The first sampling measurement value compares whether to exceed a prescribed threshold and judges the control part at the sampling time. The control part compares whether the first sampling measurement value exceeds a prescribed threshold. When the actual measurement value doesn't exceed a prescribed threshold, the control part predict the first sampling value at the next sampling time. [1] When the first sampling the predicting value doesn't exceed the threshold value, the status of the switch is maintained, [2] When the first sampling the predicting value exceeds the threshold value, the time when the movement of the switch should be changed is calculated and the status of the switch is changed at time concerned.