You need to apply for a HK visa from Chinese embassy or consulate in Japan. it takes 4 working days for visa processing, but if your application will be sent to HK immigration, it will take about 4 weeks.
The requirements are visa application form, copy of the passport or travel document including the used visa pages, Financial certificate, such as bank statement, Employment certificate, Hotel reservation and round trip air-ticket, certificate of the trip purpose, Copy of the used travel documents to HK before (if you have) and Copy of other countries' valid visa (such as your Japan residence permit).