Provided are a constant voltage circuit configured to, when a power supply voltage is low, detect a leakage current to output a stable voltage at a power supply voltage level, and a crystal oscillation circuit using the constant voltage circuit. The constant voltage circuit includes a leakage current detection circuit including a PMOS transistor for monitoring a leakage current, which has a gate and a source being grounded. When a leakage current is detected, even with a constant voltage power supply, a voltage sufficient for turning on an output transistor of the constant voltage circuit can be applied to a gate of the output transistor.