A constant temperature crystal oscillator includes on a circuit substrate: a surface-mount crystal resonator which is provided with two crystal terminals as mount terminals and a dummy terminal on the bottom surface, and has a metal cover; an oscillation circuit element which forms an oscillation circuit together with the crystal resonator; and a temperature control element which keeps a constant operation temperature of the crystal resonator, in which the temperature control element includes at least a heating chip resistor, a power transistor for supplying electric power to the chip resistor, and a temperature sensitive resistor for detecting the operation temperature of the crystal resonator, wherein a dummy terminal on the substrate side of the circuit substrate for connection to the dummy terminal of the crystal resonator is connected to a resistor terminal on the substrate side to which the temperature sensitive resistor is connected through a conductive path.