To provide a collision detecting device capable of reducing manufacturing cost without degrading collision detection accuracy.

The collision detecting device includes: a first sensor 2 which is arranged behind a first row seat in a cabin and which detects vehicle behavior relevant to collision; a second sensor 3 which are arranged lateral to the first row seat in a vehicle or forward in the cabin and which detect the vehicle behavior relevant to the collision; and a determination unit 4 determining whether or not there is the collision with the rear side surface of the vehicle based on an output signal from the first sensor 2 and the second sensor 3. The determination section 4 stores a safing threshold and a main threshold, which is larger than the safing threshold, and determines that there is the collision only when an output value from the second sensor is larger than the safing threshold and an output value from the first sensor 2 is larger than the main threshold.