A seat-plate-mounted capacitor includes a capacitor body and a seat plate. The capacitor body includes a case and a sealing member. The case includes a tubular part and a bottom wall that closes a first end of the tubular part. The sealing member closes a second end of the tubular part that is opposite to the first end. The capacitor body is disposed so that the sealing member is adjacent to the seat plate. The seat plate includes a base part and a support wall. The tubular part has an annular recess surrounding the tubular part, a first maximum diameter portion disposed closer to the bottom wall than the annular recess is, and a second maximum diameter portion disposed closer to the second end than the annular recess is. A diameter A of the first maximum diameter portion, a diameter B of the second maximum diameter portion, and an inner diameter C of the support wall satisfy a relation A>C>B, in a state that the capacitor body is not installed in the seat plate.