To provide an optical sub-assembly (OSA) which is configured by attaching a sleeve member to an optical module using a cap shell formed by press-molding by an adhesive and can secure a large adhesion area.

The OSA 1 includes a coaxial optical module 20 where a cap shell 23b which holds a condenser lens 23a and is formed by press-molding is attached to a stem 22 on which a PD 21 is mounted, and a sleeve member 10 for arraying the ferrule 51 of an optical connector 50. An extending part 40 extending above the holding surface 23c of the lens 23a of the cap shell 23b is disposed in the cylindrical outer peripheral part of the cap shell 23b, and a recess 12a is formed in the sleeve member 10 to house the cap shell 23b. The outer peripheral surface of the optical module 20 including the outer peripheral surface of the extending part 40 disposed in the cap shell 23b and the inner peripheral surface of the recess 12a of the sleeve member 10 are fixed together by an adhesive.