A collision detection device is provided with a coil which is arranged at an inner side of an outer panel of a body portion of a vehicle and faces the outer panel to generate a magnetic field in a direction toward the outer panel, and a collision determining unit for determining a collision with the body portion of the vehicle based on a variation of an inductance of the coil. The collision determining unit includes a resonance circuit which is constructed of the coil. When there occurs a collision with the vehicle body portion, a distance between the outer panel and the coil will decrease so that an inductance of the coil can instantly decrease. Therefore, a resonance frequency of the resonance circuit will increase and be detected, thus shortening the response time.