The present invention provides a modulator integrated laser device whose modulator can be differentially driven without any problem. The modulator integrated laser device includes a laser section (12) having a first lower cladding layer (42), a separation section (14) having a second lower cladding layer (60) and a modulation section (16) having a third lower cladding layer (70), which are formed on a same substrate (40), wherein the width of the second lower cladding layer (60) is narrower than the width of the first lower cladding layer (42) and the width of the third lower cladding layer (70) along the short size direction.