While maintaining a shaft in a fixed state, an outer ring is rotated. Vibrations of the shaft are detected by a first vibration pickup and a second vibration pickup. Detection signals of both the vibration pickups are delivered to a frequency converter via amplifier, respectively. The frequency converter determines, by Fourier transformation, the rotation frequency of the outer ring, the revolution frequency of balls and the resonance frequency of the double-row ball bearing. A first computing assembly then calculates the contact angle and bearing rigidity, and a second computing assembly calculates the preload.