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 .