内定承諾書を提出した後も辞退はできるし就活の継続も可能ですが、入社する可能性が低いなら内定承諾書を提出する前に辞退しましょう。 from here
I think that last section is the key--politeness and expectations. It's a case of only one, but our daughter had been accepted and confirmed that at one company, but then switched to another. So no, not legally binding, but immediately following she spent quite a bit of time on the phone with the first company explaining herself. (and left a thoroughly burned bridge behind) Still, that was maybe ~8 years ago, and the labor market is tighter now, so times may have changed.
The phrasing of your initial question might need some editing ("other companies"). This could sound like you might accept at multiple companies. In the immediately above example, it's a first company and then a second, due to a timing issue, not several (or more?).
Anyway, good luck with the job, wherever you end up!
It's not the phrasing, it's my approach to job hunting that needs editing. The thing is - when I accepted that first naitei I was a bit desperate since I really wanted to secure a working visa before my student visa expires (March 31st, 2020), but it later turned out that after going through all the preliminary tests and interviews passing that final interview is not that big a deal. So the number of naiteis began accumulating and now I don't even need to move from my apartment, not to mention 引っ越し to another prefecture 400kilometers away - the (hastily) accepted offer in question.
Anyway, thanks for sharing your thoughts and experience on the matter, really appreciate that.
Headmaster of the HS where I worked gave me a recommendation for a job at another HS, who accepted me. Then, I got accepted at university, so he advised that I write an apology explanation letter, which I did, to the HS that wanted me. I did and never looked back.