The present invention relates to a sensor mechanism of an optical disc device for detecting tray in/out motion, which comprises a swing bar having a actuating bar, a spinning center and a protrusion portion; a gear for driving the protrusion portion to rotate the swing bar against the spinning center; at least one sensor for emitting a signal that the tray has been already moved in/out when it is touched by the actuating bar, so that the device prepares to perform next function.