An inspection device includes an oscillator for generating ultrasonic waves toward a first connector, and a vibration receiver for receiving vibrations generated in the first connector. The inspection device includes a vibration controller for analyzing the vibrations received by the vibration receiver. The vibration controller detects a resonance frequency by converting, by Fourier transform, the vibrations received by the vibration receiver. The vibration controller determines an insertion amount of the first connector into the second connector, based on the detected resonance frequency.