In the present disclosure, in an EADFB laser in which an SOA has been integrated, a new configuration in which a problem of deterioration of optical waveform quality and insufficient optical output is solved or mitigated while taking advantage of characteristics that the same layer structure can be used and a manufacturing process can be simplified is shown. In an optical transmitter of the present disclosure, a waveguide structure having different core widths (waveguide widths) is adopted while using the same layer structure for a DFB laser and the SOA. Waveguides with different core widths are adopted so that a problem of insufficient saturated optical output or waveform deterioration due to a pattern effect is solved and mitigated. A passive waveguide region having a tapered shape is introduced in a part between an EA modulator and the SOA so that a waveguide width is continuously changed.