To provide a simple mounting structure of an air bag sensor which can react an air bag securely without delay even at the time of underride collision.

This mounting structure of an air bag sensor is provided with a heater unit 4 in front of the air bag sensor 1 provided on a front floor surface 5 of an automobile, and a displacement transmission member 15 which transmits a displacement input to the air bag sensor 1 according to rear displacement of the heater unit 4 at the time of front collision is provided between the heater unit 4 and the air bag sensor 1. Consequently, a shock is input into the air bag sensor 1 by the displacement transmission member 15 displaced in accordance with the rear displacement of the heater unit 4 at the time of underride collision when the shock is not directly applied on the front floor surface 5.