An optical subassembly may include a device mounting substrate on which an optical device is mounted, a relay substrate including a first conductor pattern transmitting a electrical signal to the optical device, a pedestal including a third surface on which the relay substrate is placed and a fourth surface on which the device mounting substrate is placed and a spacer interposed between the third surface and the relay substrate to electrically connect the relay substrate and the pedestal. In an optical subassembly, the first lead terminal may include a small-diameter part and a large-diameter part provided at an end of the small-diameter part and having a larger diameter than that of the small-diameter part, and at least part of the large-diameter part may be exposed from the dielectric on a first surface side and the first lead terminal and the first conductor pattern may be connected by brazing and soldering.