A semiconductor Mach-Zehnder optical modulator comprising: an optical waveguide (12; 13; 16; 17; 18; 19) formed on a substrate; at least one dielectric layer on the substrate; an input side lead-out line (20; 21; 22; 23) formed on the at least one dielectric layer, the input side lead-out line (20; 21; 22; 23) including a first end to which a modulation signal is input; a phase modulation electrode line (24; 25; 26; 27) formed on the dielectric layer along the optical waveguide (12; 13; 16; 17; 18; 19), the phase modulation electrode line (24; 25; 26; 27) including a first end connected to a second end of the input side lead-out line (20; 21; 22; 23); an output side lead-out line (28; 29; 30; 31) formed on the dielectric layer, the output side lead-out line (28; 29; 30; 31) including a first end connected to a second end of the phase modulation electrode line (24; 25; 26; 27); an electrode (32; 33; 34; 35) configured to apply a modulation signal to the optical waveguide (12; 13; 16; 17; 18; 19), the modulation signal propagating through the phase modulation electrode line (24; 25; 26; 27), characterized in that the semiconductor Mach-Zehnder optical modulator comprises: an attenuator configured to attenuate the modulation signal in one of a lower layer under the output side lead-out line (28; 29; 30; 31) and an upper layer over the output side lead-out line (28; 29; 30; 31); and a termination resistor connected to the output side lead-out line (28; 29; 30; 31) wherein the attenuator includes at least one semiconductor layer that is formed intermittently along the output side lead-out line (28; 29; 30; 31) in the lower layer under the output side lead-out line (28; 29; 30; 31), the at least one semiconductor layer being n-type or p-type. an optical waveguide (12; 13; 16; 17; 18; 19) formed on a substrate; at least one dielectric layer on the substrate; an input side lead-out line (20; 21; 22; 23) formed on the at least one dielectric layer, the input side lead-out line (20; 21; 22; 23) including a first end to which a modulation signal is input; a phase modulation electrode line (24; 25; 26; 27) formed on the dielectric layer along the optical waveguide (12; 13; 16; 17; 18; 19), the phase modulation electrode line (24; 25; 26; 27) including a first end connected to a second end of the input side lead-out line (20; 21; 22; 23); an output side lead-out line (28; 29; 30; 31) formed on the dielectric layer, the output side lead-out line (28; 29; 30; 31) including a first end connected to a second end of the phase modulation electrode line (24; 25; 26; 27); an electrode (32; 33; 34; 35) configured to apply a modulation signal to the optical waveguide (12; 13; 16; 17; 18; 19), the modulation signal propagating through the phase modulation electrode line (24; 25; 26; 27), characterized in that the semiconductor Mach-Zehnder optical modulator comprises: an attenuator configured to attenuate the modulation signal in one of a lower layer under the output side lead-out line (28; 29; 30; 31) and an upper layer over the output side lead-out line (28; 29; 30; 31); and a termination resistor connected to the output side lead-out line (28; 29; 30; 31) wherein the attenuator includes at least one semiconductor layer that is formed intermittently along the output side lead-out line (28; 29; 30; 31) in the lower layer under the output side lead-out line (28; 29; 30; 31), the at least one semiconductor layer being n-type or p-type. an attenuator configured to attenuate the modulation signal in one of a lower layer under the output side lead-out line (28; 29; 30; 31) and an upper layer over the output side lead-out line (28; 29; 30; 31); and a termination resistor connected to the output side lead-out line (28; 29; 30; 31) wherein the attenuator includes at least one semiconductor layer that is formed intermittently along the output side lead-out line (28; 29; 30; 31) in the lower layer under the output side lead-out line (28; 29; 30; 31), the at least one semiconductor layer being n-type or p-type.