To provide a detection structure of bottoming timing of a air bag capable of properly detecting the bottoming timing in the case of performing a shock test with an air bag for test.

The detection structure of bottoming timing detects the bottoming timing when the outer surface 21 and the inner surface 23 of a side air bag 3 and each inner side touch upon touching of a dummy 39 to the outer surface 21 of the expanded side air bag 3 and denting of the side air bag 3. To the inner sides of the outer surface 21 and the inner surface 23 corresponding to the part where the dummy 39 touches, conductive cloths 9a to 13b are attached and a conduction detection means detecting conduction of each other conductive cloth 9a to 13b by touching is provided.