A collision sensor adapted to sense acceleration applied thereto in the event of collision of a vehicle and actuate an occupant protective system. A rotary element has an outer peripheral surface thereof provided with a predetermined curvature characteristic. A biasing device generates a biasing force for biasing the rotary element to a predetermined reference position. An inertial mass is responsive to acceleration acting thereupon for causing the rotary element to rotate from the predetermined reference position against the biasing force. A switch is operable to generate an output signal when the rotary element has rotated by a predetermined angle from the predetermined reference position. The biasing force of the biasing means is modulated in accordance with the predetermined curvature characteristic of the outer peripheral surface of the rotary element as the rotary element rotates.
