detective agency; private investigation firm
Refers to a business office where private detectives work, handling investigations for clients.
He opened a detective agency.
I request an investigation from a detective agency.
探偵事務所 is the more common, general term for a detective agency, while 興信所 has a slightly formal, old-fashioned ring and often implies background or credit investigations.
Compound of 探偵 (detective) + 事務所 (office).