A landing gear assembly for a semitrailer includes a pair of legs that may be simultaneously raised or lowered by turning a single crank. A disturbance estimating device is based on an expansion system based on the mathematical model for estimating a disturbance of the control object in accordance with an input signal inputted into the control object and a detection signal detected from the control object. Barium and strontium are provided within the reactor by flowing at least one metal organic precursor to the reactor.