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"Why does the stock price drop even after an acquisition?" — A super introductory guide to solving M&A questions

2026年4月9日 06:39

コスモス：買収ニュース見て飛び乗ったら、
        その日に含み損になったことない？
        「なんで買収したのに株価下がるのー？」って。

エール  ：あるある。いいニュースのはずなのに、
        なんで暴落するんだろう。

コスモス：今日はそのモヤモヤを解決したいと思います。

Today is a super introductory guide to M&A.
This article breaks it down to a level that even beginners can understand.

The trigger was Buffett's "goodwill"

I previously wrote an article explaining a book about Buffett.

As expected of Mr. Buffett, it was very well received. Thank you to everyone who read it 🌸

In the section on "owner earnings," the term goodwill came up.

エール  ：あの「のれん」？ラーメン屋の？

コスモス：違うの（笑）。M&Aのときに発生する「のれん」のこと。
        バフェットは「会計上ののれん」と「経済的なのれん」は
        別だって言ってたんだけど、要するに

       「本来の価値より高く買った差額」のことなんだよね。

        でもそもそも「のれん」ってどうやって生まれるんだろう？
        って気になって。それでM&Aをちゃんと調べてみたんだ。

エール  ：へえ。それで何がわかったの？

コスモス：「なんで買収したのに株価下がるの？」って
        疑問の答えが見えてきたんだ。

エール  ：それ、めっちゃ知りたい。

M&A = A corporate marriage

Simply put, M&A is when one company joins with another.

It is an abbreviation for Mergers and Acquisitions.

For example, suppose you are running a ramen shop.

You want to rent the vacant store across the street to start a cafe, but you don't really know how to run a cafe.

So, you acquire an already popular cafe chain.

This is M&A.

エール  ：自分でゼロから始めるより、確かに早いね。

コスモス：そう。時間を買うっていう考え方が近いかな。
        でも正直さ、それってめちゃくちゃ楽そうに見えるじゃん？

エール  ：そんな簡単な話じゃないの？

コスモス：実は落とし穴があるんだよ。

Let's use the marriage analogy here.
Marriage doesn't end once the ceremony is over, right?

Rather, that is when life begins.

エール  ：たしかに…そこからが本番だよね。
コスモス：M&Aも同じなんだ。
        会社を買うのは「結婚式」みたいなもの。
        一番大事なのは、その後どうやって一緒に暮らしていくか。

PMI = Adjusting to married life

In the world of M&A, post-acquisition management integration is called "PMI."

エール  ：また難しい言葉…。
コスモス：簡単に言うと、
        結婚生活をどうやって回していくかってこと。

For example, imagine a couple who just got married.

They were kind at first, but before you know it, the number of times they ask, "Why didn't you do this?" starts to increase.

Even though they were supposed to be considerate of each other, before they know it, "You should know without me saying it, right?" happens more often.

If you leave small discomforts unaddressed, one day they will all explode at once.

エール  ：うわ…なんかリアルだね。。。

コスモス：でしょ？会社もまったく同じなんだ。

Work styles ("Just try it first" group vs. "Plan before acting" group)

Approach to evaluation systems (Results-oriented vs. Process-oriented)

Internal unspoken rules (such as timing for reports or the vibe of meetings)

These are all different for every company.
Aligning these is what PMI is all about.

コスモス：一緒に住み始めたら、話し合ってルールを決めるでしょ？
        でも実際は、「言い出しづらいこと」もたくさんある。

エール  ：でもそこをもしサボったら…ちょっと怖いね。

コスモス：最悪、離婚するかもしれないよね。

Companies are the same way.

Talented employees end up quitting
⇒ "That person has become kind of cold, haven't they?"

Lack of coordination
⇒ You gradually stop hearing what the other team is doing.

Not achieving the expected results
⇒ The initial enthusiasm cools down, and it just becomes an "obligation."

The company you went to the trouble of buying doesn't function at all.
This is the true nature of an "M&A failure."

3 points investors should look at

When you see M&A news, try checking these three things.

1. Is the price too high?

Whether the acquisition price is appropriate. If you buy at a high price, there is a risk that it will become a loss later in the form of "goodwill impairment."

2. Is it really effective?

Is there a synergy created by joining together? Acquisitions done "just because" usually fail.

3. Will the integration go well?

Is there a system in place to properly manage "married life" after the purchase?
This is the most important, and yet the most overlooked, aspect.

エール  ：ねえ、結局どうすればいいの？

コスモス：まずは、M&Aのニュースを見たときに、
        これを意識して考えてみて。

Focus not on "the purchase," but on "what happens afterward."

コスモス：投資ってね、“発表された瞬間”じゃなくて、
        “その後に何が起きるか”にお金がつくんだよ。

        これだけで、ニュースの見え方ってほんと変わるから。

Next time preview

Next time, using actual corporate case studies,

"Companies that failed at marriage and companies that succeeded"

I would like to look at these specifically. Stay tuned! 🌸

(Update) Posted on April 10, 2026!

Thank you for reading until the end today as well🌸

[Breaking News] Lightning flashed brilliantly today, April 9th!!

Just yesterday (April 8th),
I posted an article explaining "the moment lightning flashes"🌸

いいなと思ったら応援しよう！

ここまで読んでくれてありがとうございます🌸 さらに「次も読んでみたい」「かぶたねや創作をもっと育ててほしい」と思ってもらえたら、チップでの応援は、これからのnote活動に大切に使わせていただきます😊