An oven controlled crystal oscillator capable of uniformly transmitting heat from the heat generator to improve the frequency-temperature characteristics. The oven controlled crystal oscillator includes a high thermal conductivity plate having high thermal conductivity and provided on one side of a substrate, where the crystal resonator is provided, in such a manner to contact the resistors, the transistor, the crystal resonator, and the temperature sensor. This structure can transmit heat from the resistors and the transistor as the heat generator to the crystal resonator and the temperature sensor rapidly with less heat loss to assure a uniform temperature inside the substrate, thereby improving the frequency-temperature characteristics.