In a modulator integrated laser 100, a wave guide channel 200a in a modulator area 200 having a core constituted of an absorption layer 204 and a wave guide channel 250a at a laser area 250 having a core constituted of an active layer 254 are optically connected with each other by a wave guide channel 150a having a core constituted of a light wave guide layer 204b. The width of the light wave guide layer 204b increases in a continuous manner as the distance from the modulator area 200 increases and the width also increases in a continuous manner as the distance from the laser area 250 increases, with the width achieving its maximum in the vicinity of the middle area between the modulator area 200 and the laser area 250.