To enable to prevent occurrence of clipping by detecting this clipping from a variable capacitance sensor output by a control circuit to operate for avoiding when the clipping occurs.

Variable capacitance sensors 11, 12 are placed at a front and back of an upper edge of a window frame, a level of both output S11, S12 of a phase discriminating circuit 20, 21 are an L, and a control circuit 2 checks no approach of a substance with high specific inductive capacity (human body) in normal operation. For example, capacity of the variable capacitance 12 increases as a human body approaches to a neighborhood of the variable capacitance 12. Then, delay amounts of a variable delaying circuits 16, 17 are significantly reversed and an output S8 slightly delays from an output S7. The control circuit 2 inputs an H level of an output S11. Also, a difference of delay amounts between variable delaying circuits 18, 19 expands and the control circuit 2 inputs an L level of output S12. As a result, the control circuit 2 checks a presence of human body in the neighborhood of the variable capacitance sensor 12 near the upper edge of the window frame to instruct a window driving circuit to avoid clipping the human body.