A hinge mechanism includes two pivot shafts substantially parallel to each other, two main gears non-rotatably respectively sleeved on the pivot shafts, a transmission assembly positioned between the two pivot shafts to transmit a torque of one of the two main gears to the other in a reverse direction, and a connecting member defining two shaft holes. The pivot shafts are respectively rotatably received in the shaft holes. Two contact assemblies are respectively mounted on the pivot shafts abutting the connecting member. One of the connecting member and each main gear forms at least one detent, and the other defines at least one recess correspondingly. The at least one detent is received in the at least one recess.