Abstract: A pair of capacitors are provided at a window sash or a window glass edge. Each capacitor is connected to two variable phase delay generators in parallel. Selectors are inserted between the capacitors and variable phase delay generators. Each selector alternates the connection between the capacitor and two variable phase delay generators. Upon insertion of human bodies, two variable phase delay generators reverse relative phase delays of the output signal from the two variable phase delay generators. If one of the relative phase delays is reversed, the human bodies are detected to be inserted into the window opening.