An optical transmitter unit, a method of connecting an optical transmitter module and a transmitter side optical connector, and an endoscope system which reduce a light loss in optical transmission are provided. An optical transmitter unit 10 is configured in such a manner that a transmitter side optical connector 20 and an optical transmitter module 30 are coupled and fixed by a fixing member. The transmitter side optical connector 20 includes a ferrule 22 that holds an optical fiber 21, and includes a flange portion 23. The optical transmitter module 30 includes a light emitting element 32 and a metal case 34. A connector side screw portion 24 is provided on the ferrule 22. A module side screw portion 37 formed of an elastic member and configured to be screwed with the connector side screw portion 24 is provided in a sleeve 35. When the transmitter side optical connector 20 and the optical transmitter module 30 are fixed by the fixing member, the module side screw portion 37 is elastically deformed by pressing force of the fixing member.
