Looking at this company from the perspective of an employee at the head office, it is actually quite good. The pay is higher than elsewhere, and there is a salary adjustment every year, with fairly substantial increases (based on what I have heard, even publicly listed companies and foreign companies around me do not offer increases this large). The work itself is not particularly difficult either. It feels like a company where the longer you stay, the higher your salary becomes (perhaps because of Japanese corporate culture?). The atmosphere among colleagues is good, and the organization is fairly flat, with managers who are not overly formal or intimidating. Some departments hold meetings, but there are almost no meetings that feel like a waste of time. There is not the overtime culture I imagined; most people leave work on time, and the average age of employees is quite young.
The company's way of doing things is very outdated. The paper-based processes that cannot be eliminated result in a heavy workload. Also, most internal employees speak Japanese, but their level of professionalism is relatively low. So, if you are highly specialized in a particular area, you may either be given significant responsibility here—or find yourself wanting to bang your head against the wall. In addition, Japanese people tend to be more conservative and dislike change, so you should be mentally prepared for that.