A pre-equalization optical transmitter, which uses an RZ method and an NRZ method as a method of transmission by switching from one method to another, comprising: a tap coefficient selecting unit configured to save in advance a tap coefficient for generating an RZ pre-equalization signal which is used in the RZ method and a tap coefficient for generating an NRZ pre-equalization signal which is used in the NRZ method, and select and output one of the tap coefficient for generating the RZ pre-equalization signal and the tap coefficient for generating the NRZ pre-equalization signal according to a value of a parameter that is a factor of fluctuations in transmission power; a pre-equalization signal generating unit configured to generate the RZ pre-equalization signal when the tap coefficient output from the tap coefficient selecting unit is the tap coefficient for generating the RZ pre-equalization signal, and generate the NRZ pre-equalization signal when the tap coefficient output from the tap coefficient selecting unit is the tap coefficient for generating the NRZ pre-equalization signal; a digital-analog conversion unit (3, 4) configured to convert the RZ pre-equalization signal or the NRZ pre-equalization signal output from the pre-equalization signal generating unit into an analog signal; an amplifying unit (5, 6) configured to amplify the analog signal output from the digital-analog conversion unit (3, 4); a light source configured to output an optical signal; and a modulating unit configured to generate a pre-equalization optical transmission signal by modulating the optical signal input from the light source based on the amplified analog signal which is output from the amplifying unit (5, 6), wherein the tap coefficient selecting unit is configured to switch the tap coefficients according to the value of the parameter, thereby switching the method of transmission between the RZ method and the NRZ method during transmission. a tap coefficient selecting unit configured to save in advance a tap coefficient for generating an RZ pre-equalization signal which is used in the RZ method and a tap coefficient for generating an NRZ pre-equalization signal which is used in the NRZ method, and select and output one of the tap coefficient for generating the RZ pre-equalization signal and the tap coefficient for generating the NRZ pre-equalization signal according to a value of a parameter that is a factor of fluctuations in transmission power; a pre-equalization signal generating unit configured to generate the RZ pre-equalization signal when the tap coefficient output from the tap coefficient selecting unit is the tap coefficient for generating the RZ pre-equalization signal, and generate the NRZ pre-equalization signal when the tap coefficient output from the tap coefficient selecting unit is the tap coefficient for generating the NRZ pre-equalization signal; a digital-analog conversion unit (3, 4) configured to convert the RZ pre-equalization signal or the NRZ pre-equalization signal output from the pre-equalization signal generating unit into an analog signal; an amplifying unit (5, 6) configured to amplify the analog signal output from the digital-analog conversion unit (3, 4); a light source configured to output an optical signal; and a modulating unit configured to generate a pre-equalization optical transmission signal by modulating the optical signal input from the light source based on the amplified analog signal which is output from the amplifying unit (5, 6), wherein the tap coefficient selecting unit is configured to switch the tap coefficients according to the value of the parameter, thereby switching the method of transmission between the RZ method and the NRZ method during transmission.