An acceleration detector including a mass member and a fixed contact, which keep contact with each other by an elastic member that presses the mass member to the fixed contact. When a vehicle undergoes an impact from its left side in its traveling direction, for example, and the mass member receives inertial force that exceeds the pushing force of the elastic member, the mass member moves toward the elastic member, thereby separating from the fixed contact. By providing the mass member and the fixed contact on both sides of the elastic member, side-to-side impact applied to the vehicle can be detected. The acceleration detector with such a configuration can detect the impact acceleration at high response sensitivity and in both directions.