A sanitary washing device configured to be mounted on a toilet, comprising: a toilet seat; a nozzle configured to discharge water toward an ano-genital region of a human body; a valve unit provided on a pipe line between a water supply source and the nozzle, the valve unit including an electromagnetic valve; a controller configured to control operation of the nozzle and the valve unit; a heat exchanger unit provided downstream of the valve unit on the pipe line, and heating water supplied from the water supply source; and a casing storing the nozzle, the valve unit, and the controller, the valve unit being disposed further frontward than the controller in a longitudinal direction of the sanitary washing device, the heat exchanger unit being disposed between the controller and the valve unit in the longitudinal direction of the sanitary washing device, wherein the casing includes a low portion positioned below the toilet seat, and a high portion positioned backward of the low portion, a length in a vertical direction of the high portion being greater than a length in the vertical direction of the low portion, the valve unit is disposed in the low portion, an upper surface of the low portion becomes lower from a back toward a front, and an upper end of the valve unit becomes lower from a back toward a front, characterized in that a length in a vertical direction of the valve unit is less than a length in a longitudinal direction of the valve unit, and less than a length in a lateral direction of the valve unit, wherein the length in the vertical direction of the valve unit is a distance from the lower end of the lowermost part included in the valve unit to the upper end of the uppermost part included in the valve unit; the length in the longitudinal direction of the valve unit is a distance from the front end of the frontmost part included in the valve unit to the back end of the backmost part included in the valve unit; and the length in the lateral direction of the valve unit is a distance from the right end of the rightmost part included in the valve unit to the left end of the leftmost part included in the valve unit, the casing includes a drain path on its inner bottom surface for draining water into the toilet, the drain path does not overlap the controller in the vertical direction, and at least a portion of the drain path overlaps the valve unit in the vertical direction, at least a portion of the drain path overlaps the heat exchanger unit in the vertical direction, and at least a portion of the drain path overlaps the nozzle in the vertical direction. a toilet seat; a nozzle configured to discharge water toward an ano-genital region of a human body; a valve unit provided on a pipe line between a water supply source and the nozzle, the valve unit including an electromagnetic valve; a controller configured to control operation of the nozzle and the valve unit; a heat exchanger unit provided downstream of the valve unit on the pipe line, and heating water supplied from the water supply source; and a casing storing the nozzle, the valve unit, and the controller, the valve unit being disposed further frontward than the controller in a longitudinal direction of the sanitary washing device, the heat exchanger unit being disposed between the controller and the valve unit in the longitudinal direction of the sanitary washing device, wherein the casing includes a low portion positioned below the toilet seat, and a high portion positioned backward of the low portion, a length in a vertical direction of the high portion being greater than a length in the vertical direction of the low portion, the valve unit is disposed in the low portion, an upper surface of the low portion becomes lower from a back toward a front, and an upper end of the valve unit becomes lower from a back toward a front, characterized in that a length in a vertical direction of the valve unit is less than a length in a longitudinal direction of the valve unit, and less than a length in a lateral direction of the valve unit, wherein the length in the vertical direction of the valve unit is a distance from the lower end of the lowermost part included in the valve unit to the upper end of the uppermost part included in the valve unit; the length in the longitudinal direction of the valve unit is a distance from the front end of the frontmost part included in the valve unit to the back end of the backmost part included in the valve unit; and the length in the lateral direction of the valve unit is a distance from the right end of the rightmost part included in the valve unit to the left end of the leftmost part included in the valve unit, the casing includes a drain path on its inner bottom surface for draining water into the toilet, the drain path does not overlap the controller in the vertical direction, and at least a portion of the drain path overlaps the valve unit in the vertical direction, at least a portion of the drain path overlaps the heat exchanger unit in the vertical direction, and at least a portion of the drain path overlaps the nozzle in the vertical direction.