The deadline for application is the end of January. Internships in japan are arranged with companies such as Omron Corporation, Olympus, Nippon Paper, Mitsubishi, Ricoh, Tokyo Electric Power Co., etc. Citizens of any country may participate. International Computer Professional Associates assists multinational companies to recruit technology, marketing, and finance professionals for jobs in Japan, Hong Kong, Singapore, China, and the US Pacific coast. International Cooperative Education Program offers internships that pay well enough to cover in-country costs.
Interns are responsible for their own airfare.
Citizens of most countries are eligible. International University of Japan Career Counseling and Services has information on internships in Japan. You can't apply through IUJ unless you are a student there, but they give contact information for companies that sponsor interns which you can then contact on your own. The service is primarily for students who live in Japan or who are returning to their family during the summer months. These jobs do not provide housing so are best suited for students who have a place to stay.