To provide a new CI measuring method for a crystal oscillator which can easily and speedily measure the CI of the crystal oscillator belonging to a high-frequency area, with high precision and a new crystal oscillation circuit which can obtain a highly-stable high-frequency output.

The DC input voltage VAGC of the crystal oscillation circuit is measured as the CI of the crystal oscillator XTAL1, using the crystal oscillation circuit equipped with an integrating circuit composed of a transconductance circuit OTA1 and a high-output/low-input impedance converting circuit AV1 connected to its output side at its output stage and one or more AGC amplifying circuits GV1, having an amplification factor corresponding to the DC input voltage VAGC between the crystal vibrator XTAL1 and integration circuit.