A plate-shaped circuit board configured to receive a detection signal from a detector is housed in a case and is held by a holder with a flat substrate surface thereof intersecting with a bottom surface of a cover member. The holder is attached to the cover member. The cover member includes a cover body, and elongated supporting portions projecting from the bottom surface of the cover body into the case. The supporting portions each include a pair of support projections that support one of opposite side surfaces of the circuit board along a projecting direction of the supporting portions. The holder includes a flat portion provided with engagement pieces engageable with a second substrate.