An internship in Japan is an investment into your professional future
Internships are usually done by students or graduates to get practical work experience, applying the theory they learn at university in a real life work environment. Many students want to do an internship abroad to expand their global and intercultural skills.
Unfortunately, the idea of doing a student internship is not very common in Japan. Japanese students, while attending university usually don’t aim at getting practical work experience, but they learn about real work only after graduation when entering a company at an entry-level position. It is still the idea of many of them to stay with the same company thoroughout their professional life. As a consequence, many Japanese companies, particularly those with a more traditional mindset, don’t see the point why a student wants to be with them for a limited duration of time only, and they don’t see the reason why to spend effort into training someone who would leave after short time anyway.
As a consequence, it can be really difficult to arrange an internship in Japan.
There are agencies that arrange internships in Japan, but you need to expect to pay a relatively high amount of money to use their services. Also, internships in Japan are often not paid, or companies only pay a small living support such as reimbursing your commuting or accommodation costs. With the visa categories issued for internships (e.g. Cultural Activities Visa) it is also legally not allowed to have an income in Japan.
It is usually not possible to finance your trip, living expenses and possibly ageny placement fees with the stipend you get for your internship. You should therefore see an internship in Japan as an investment into your professional future.