personal reference letter; personal guarantee document
A formal document used in Japan, often required for employment or rental applications, where a guarantor vouches for the applicant's identity and character.
A personal reference letter is required when renting an apartment.
Please submit a personal guarantee document when applying for the job.
保証人 is the guarantor (person), while 身元保証書 is the written document they provide.
Compound of 身元 (identity, background) + 保証 (guarantee) + 書 (document).