A liquid delivery device includes a constant flow valve, a pump and channels. A medicinal solution bag is connected to the liquid delivery device. The pump has a suction aperture, a discharge aperture and check valves. The constant flow valve includes a valve casing and a diaphragm that partitions the interior of the valve casing to form a first valve chamber and a second valve chamber. A first opening, a second opening, and a third opening are provided in the valve casing. A conical spring arranged between and in contact with a top plate and the diaphragm is provided in the second valve chamber. The spring applies a pressure towards an O-ring side to a second main surface of the diaphragm.