To obtain a method for measuring the difference of the propagation delay time of an optical fiber capable of adapting to a parallel transmission system and automatically obtaining the difference of propagation delay time in a simple circuit constitution.

A double optical pulse generating circuit is constituted by including a differential switching circuit and lasers LD1 and LD2. The differential switching circuit includes planar-type transistors Tr1 and Tr2 to output mutually complementary electric pulse signals. The lasers LD1 and LD2 receive the electric pulse signals outputted from the transistors Tr1 and Tr2 and output (emit light) two mutually complementary optical pulse signals, and the two optical pulse signals become the output of the double optical pulse generating circuit.