A temperature control circuit of an oven controlled crystal oscillator is a circuit in which a supply voltage is applied to a first terminal of a resistance R10, a first terminal of a resistance R11, and a first terminal of a heater resistance RH1, a second terminal of a thermistor TH1, a second terminal of a temperature sensing element, and a collector side of a transistor Q2 are connected to a common ground, a first terminal of a temperature sensing element ZZ is connected to an input terminal of an operational amplifier IC2, and a setting temperature in a thermostatic oven is controlled to gradually rise as an ambient operative temperature rises.