An optical semiconductor device includes a chassis that has an external wall, a feedthrough that penetrates the external wall of the chassis and has a projection portion projecting toward outside of the chassis from the external wall, a connection terminal that is electrically connected to a component mounted in the chassis and is on the projection portion of the feedthrough, a first temperature detector that is on an external face of the external wall of the chassis and detects a temperature of the chassis, and a flexible substrate of which an end is connected to the connection terminal and of which a portion spaced from the end is connected to the first temperature detector, wherein the first temperature detector is between the external wall and the flexible substrate.