Abstract: The present invention relates to an automatic detector for a control device for controlling the flying state of a remote-control toy airplane, which can provide automatic correction to maintain the remote-control toy airplane in a flying state in a level or horizontal manner. The detector is a digital or analogic device located at a proper position within the housing of the airplane. The detector includes a device locating plate, fixing racks, a suspension cord, a movable light-obstructing sheet or a movable light-permeable sheet, light emitting element, and one or more detecting elements the light-obstruction sheet is swingingly mounted on the suspension that in turn is connected to the cord fixing racks. When light emitting element emits light and the airplane maintains a horizontal or level attitude, the effect of gravity on light-obstruction sheet will cause it to obstruct light.