A wavelength tunable laser comprising: a gain region including an active optical waveguide optically connected to a passive optical wave guide ; and a wavelength tunable region the wavelength tunable region including a lower clad layer , the passive optical waveguide disposed on the lower clad layer , an upper clad layer disposed on the passive optical waveguide , an electrode disposed on the upper clad layer , a current blocking layer disposed on the electrode , a heater disposed on the current blocking layer , a first insulating groove and a second insulating groove which are disposed opposite to each other with the passive optical waveguide therebetween and a wavelength tunable filter, characterized in that the lower clad layer includes a protrusion portion formed between the first insulating groove and the second insulating groove such that the first and second insulating grooves are not connected to one another, and wherein heat transferred to the protrusion portion is blocked by the first insulating groove and the second insulating groove, wherein a refractive index of the passive optical waveguide is changed when heat is input by the heater , wherein the protrusion portion has a first protrusion portion having a width which decreases in a direction from the upper clad layer to the lower clad layer , and a second protrusion portion having a width which increases in the direction, and wherein, in the protrusion portion, a ratio between a first region having the greatest width (W1) and a second region having the smallest width (W2) in the protrusion portion is 1:0.1 to 1:0.5; and the first region having the greatest width is an upper surface of the first protrusion portion, and the second region having the smallest width is a boundary between the first protrusion portion and the second protrusion portion. a gain region including an active optical waveguide optically connected to a passive optical wave guide ; and a wavelength tunable region the wavelength tunable region including a lower clad layer , the passive optical waveguide disposed on the lower clad layer , an upper clad layer disposed on the passive optical waveguide , an electrode disposed on the upper clad layer , a current blocking layer disposed on the electrode , a heater disposed on the current blocking layer , a first insulating groove and a second insulating groove which are disposed opposite to each other with the passive optical waveguide therebetween and a wavelength tunable filter, characterized in that the lower clad layer includes a protrusion portion formed between the first insulating groove and the second insulating groove such that the first and second insulating grooves are not connected to one another, and wherein heat transferred to the protrusion portion is blocked by the first insulating groove and the second insulating groove, wherein a refractive index of the passive optical waveguide is changed when heat is input by the heater , wherein the protrusion portion has a first protrusion portion having a width which decreases in a direction from the upper clad layer to the lower clad layer , and a second protrusion portion having a width which increases in the direction, and wherein, in the protrusion portion, a ratio between a first region having the greatest width (W1) and a second region having the smallest width (W2) in the protrusion portion is 1:0.1 to 1:0.5; and the first region having the greatest width is an upper surface of the first protrusion portion, and the second region having the smallest width is a boundary between the first protrusion portion and the second protrusion portion. <EOS>