PURPOSE: To provide an integrated type semiconductor device having a fail-safe function and an optical communication network using it.

CONSTITUTION: A double light wave guide structure by layering a light wave guide containing a first core layer 1-1-3 and a light wave guide containing a second core layer 1-1-6, is provided on a semiconductor substrate 1-1-1. An integrated type semiconductor device is divided into plural areas 1, 2 and 3 in the wave guide direction. At least a single area 2 among the areas 1, 2 and 3 has a refractive index modulating layer 1-3-1 to control the light distribution of the double light wave guide structure. In respective ones of the plural areas 1, 2 and 3, electrodes 1-2-2, 3 and 4 are provided. In a condition where external control is not applied to the area 2 having the refractive index modulating layer 1-3-11, a first wave guide and a second wave guide are constituted so as not to be joined together, and have a fail-safe function.