In a side door collision detecting system for a vehicle, an internal pressure sensor detects a door internal pressure of a side door of the vehicle. The internal pressure sensor outputs an internal pressure signal based on the door internal pressure, and the internal pressure sensor is installed in an interior space of the side door. A physical value sensor detects a physical value and outputs a physical value signal based on the physical value. The physical value sensor is installed in the interior space of the side door. A collision determining circuit determines whether the side collision occurs based on the internal pressure signal and the physical value signal.