Composite optical semiconductor device

A light emitting element and a light detecting element are formed in one body. The light emitting element has PN-junction between a first region (13/15) having first conductivity type and a second region (17) having second conductivity type in a semiconductor substrate. The light emitting element emits a light by applying a voltage between a first and a second electrode (18, 19) which are connected to the first and second region, respectively. A third region (20/21/22) is formed on the second region (17) and constitutes a part of the light detecting element together with the second region (17). The light detecting element detects a part of the emitted light and has a main current path between a third and a fourth electrode (23, 19) which are connected to the second region (17) and the third region (20/21/22), respectively. The main current path of the PD is led substantially independently of the main current path between the third and fourth electrodes (23, 19).