Abstract: An automatic temperature control circuit of a laser diode solves a problem that a temperature control becomes unstable when switching to heat or cool the laser diode. In the circuit, a peltier element changes the temperature of the laser diode by heating or cooling, a temperature detecting section detects the temperature of the laser diode and outputs the corresponding detected voltage, an error amplifying section outputs an error voltage by comparing the detected outputted voltage with a fixed value, a peltier current control section controls the direction of a driving current flowing to the peltier element in correspondence with an error voltage outputted from the error amplifying section. In addition, there is provided a circuit which feed-backs a voltage corresponding to the driving current flowing to the peltier element to an input side of the error amplifying section and adds it to the detected voltage outputted from the temperature detecting section.