To provide a circuit device suitable for incorporating a semiconductor element emitting or receiving short-wavelength light. The circuit device includes a casing, a semiconductor element, and a cover portion. The casing has an opening on the top face thereof. The semiconductor element is incorporated in the casing and emits or receives light. The cover portion is made of a material transparent to the light and covers the opening. In the periphery of the opening, a concave portion is provided, and a portion of the cover portion with a certain thickness on the bottom side is accommodated in the concave portion. Since the portion of the cover portion with the certain thickness on the bottom side is accommodated in the concave portion provided in the upper portion of the casing, the position of the cover portion is accurately fixed. Accordingly, it is possible to obtain accurate relative positions of the semiconductor element accommodated within the casing and the cover portion.