PURPOSE: To ensure the sufficient input phase margin of an electric/optic conversion means by supplying a 2nd clock signal via a phase locked loop to the electric/optic conversion means.

CONSTITUTION: An optic/electric means 111 converts an introduced optical signal into an electric signal, outputs a parallel data, extracts a 1st clock signal and outputs it, and supplies the 1st clock signal to an electric/optic conversion means 141 as a 2nd clock signal via a phase locked loop 131. The electric/optic conversion means 141 fetches a data supplied from a data input/output means 121 and the optic/electric means conversion 111 synchronously with the 2nd clock signal and converts the signal into an optical signal. Thus, the similar tendency in the phase fluctuation with respect to a change in the environment (power voltage or operating temperature or the like) of the 2nd clock signal and the data supplied via the data input/output means 121 is obtained. Thus, the input phase margin of the electric/optic conversion means 141 is ensured.