Provided herein is an external-cavity type wavelength tunable laser including gain medium configured to generate an optical signal and amplify the generated optical signal based on a bias current applied; an external reflector configured to be coupled optically with the gain medium; a second thermistor provided on the side of the gain medium and configured to measure a temperature of the gain medium; a first thermistor provided on the external reflector and configured to measure a temperature of the external reflector; and a thermoelectric cooler configured to transfer generated heat based on the temperatures measured by the first and second thermistors.