The present invention relates to a rotary switch assembly for an air conditioner in a vehicle, in which detent means is mounted on a holder joined to a front housing to mount a knob on the holder, so that a cable switch assembly for operating a cable to control the air conditioner can be used to various switches in common without regard to existence and nonexistence of a detent structure or positions of the detent structure and the rotary switch assembly can prevent a movement of the knob and effectively transfer a manipulation force of the knob to the cable switch assembly since the detent means is mounted on the holder adjacent to the knob.