PURPOSE: To generate a short optical pulse without a pattern effect, by superimposing a modifying current, which is smaller than the current value when an RZ pulse current is '1', on the RZ pulse current when it is a '0', and driving a semiconductor laser with a newly formed RZ pulse current.

CONSTITUTION: When an RZ pulse current Isa is '1', an optical pulse is generated, and a carrier density is rapidly decreased. When the current Isa is a '0', the optical short pulse is not generated. Therefore the carrier density N is not decreased rapidly. The relationship with the current value is set at a suitable value for the rapid decrease of the carrier density N at the time of the '1'. Thus even if the short optical pulse is not generated at the '0' value, the carrier density N is made to be Ni at the rising time point of the next RZ current pulse. Therefore, the carrier density N at the rising time point of the RZ pulse both for the values of '0' and '1' always becomes a constant value Ni. A previous modulating pattern Ip does not depend on the value of '0' or '1'. Thus the short optical pulse having the constant position, width and amplitude is obtained.