The present invention provides an optical subassembly that reduces the fluctuation of the optical power due to an external force applied via the optical connector mated with the subassembly. The subassembly provides an optical device, a stub and a joint sleeve. The optical device includes a laser diode (LD), a stem, a lens cap, and a lens. The interval between the optical device and the lens is set greater than a reference interval and the focal length of the lens. The reference interval is set such that, when the interval between the LD and the lens and the interval between the lens and the stub are varied to achieve the maximum optical coupling therebetween at the reference interval between the LD and the lens.