PURPOSE: To enhance light detecting efficiency and to monitor oscillating threshold condition accurately, by electrically and optically dividing active regions, clad regions and electrodes formed on the upper parts of the clad regions into a light emitting region and light receiving region, and using the electrode of the semiconductor substrate as a light reflecting mirror having high light reflectivity.

CONSTITUTION: Active regions 3a, 3b, clad regions 4a, 4b and electrode forming layers 5a, 5b are sequentially formed on a semiconductor substrate 2, and divided into a light emitting region and a light receiving region by an electrical and optical insulating groove 9. When a ground potential is applied to an N side electrode 1, an appropriate positive potential P is applied to a P type electrode 6a, and an appropriate negative potential is applied to a P side electrode 6b, the active region 3a operates as a planar stripe type semiconductor element and the active layer 3b operates as a surface light receiving type monitoring light detector. The electrodes form excellent ohmic contact with the semiconductor substrate 2 comprising N type InP under the low alloy temperature conditions such as about 300°C, and a back surface reflecting mirror having the light reflectivity of about 100% is obtained.