A collision detecting device for automobiles or the like and adapted to produce, upon detection of a collision, an electric signal for actuating a safety device such as an air-bag. The collision detecting device comprises a housing closed at its one end, an end plate closing the end of the housing and provided with a terminal, a wheel having a lapping portion and a mechanism for bringing electrical contacts into and out of contact with each other, a block supporting one end of the shaft of the wheel and fixing the contacts, electrical contacts normally kept away from the contact portion of the terminal but are moved into contact with the latter in the event of a collision, a spring adapted to normally bias the wheel into contact with the projection of the block, a lid provided with a notch for fitting a cylindrical tab of the block and adapted to cover the wheel and the spring, a spring disposed between the lid and the closed end of the housing and adapted to press the lid, and the collision detecting device of the invention can easily be assembled by automatic assembling machines.