A power on/off system of a thin projector including an optical system arranged in a housing with a panel shape having lateral sides and a front side having an area larger than an area of each lateral side is disclosed. The power on/off system includes a door, fans which cool a lamp, a shutter which opens/closes a projection lens, a first sensing unit which senses malfunctions of the lamp, the fans, the door, and the shutter, a second sensing unit which senses an internal temperature of the housing, and a controller which sequentially controls, in response to power-on of the thin projector, operations of opening the door and the shutter, driving the fans, driving the lamp, and sequentially controls, in response to power-off of the thin projector, operations of turning off the lamp, driving the fans, turning off the fans, and closing the door and shutter.