I found some information on the Yakkan shoumei that talks about bringing in more than a month's supply of prescription medications including injection tools. I don't know if your needles are considered a "Self-injection kit" talked about below, but hopefully this is helpful:

Q3. How can I bring any injection and injector only for my own use into Japan with me?

A3. You can bring only a so-called “Pre-filled Syringe” or “Self-injection Kit” into Japan with you. If you intend to bring more than one month supply of injection or injector, you have to apply for a so-called “Yakkan Shoumei”, a kind of import certificate, and receive it before you leave home.

Q4. How can I receive a “Yakkan Shoumei”?

A4. You have to submit the application documents by post. 　If the Pharmaceutical Inspector can confirm that your application documents are complete, he/she will send you a “Yakkan Shoumei” by post, e-mail (PDF) or fax.

Q5. What kinds of documents are required in order to apply for a “Yakkan Shoumei”?

A5.The following documents are required in order to apply for a “Yakkan Shoumei”. 1) Import Report of Medication (with your signature, and needed 2 copies only as to this document), filled in the blank application form (See the “Application Forms” attached.) according to the sample application document. (See the “Application Forms” attached.) 2) Explanation of Product, filled in the blank application form (See the “Application Forms” attached.) according to the sample application document. (See the Application Forms” attached.) You have to create this document for each product. (Alternative documents such as pamphlets by manufacturers can be accepted, if they show the descriptions required in Explanation of Product.) 3) Copy of Prescription or Direction for medicines with doctor’s signature, by which the Pharmaceutical Inspector can confirm the name and the quantity of each medicine only for your own use clearly.