A pump comprising: a pump section comprising a pump chamber that suctions liquid by expanding a capacity and discharges liquid by contracting the capacity; a suction section for suctioning liquid into the pump section; a discharge section for discharging liquid from the pump section; a suctioning-side check valve that suppresses a backflow of liquid from the pump section to the suction section; a discharge-side check valve that suppresses a backflow of liquid from the discharge section to the pump section; and a communication portion (6b, 60b) that provides continuous communication between the pump section and the discharge section in a state in which the discharge-side check valve suppresses the backflow, wherein a size of the communication portion is set so that, when the continuously expanding and contracting pump section contracts, a volume of gas mixed in liquid in the pump section is contracted; the discharge-side check valve is arranged on an upper side of the suctioning-side check valve; characterised in that the communication portion is provided on the discharge side check valve; and in that the pump section is provided with: a first channel having a fixed capacity with an upper end portion connected to the discharge-side check valve and a lower end portion connected to the suctioning-side check valve; and a second channel having a fixed capacity with one end portion connected to the first channel and the other end portion connected to the pump chamber. a pump section comprising a pump chamber that suctions liquid by expanding a capacity and discharges liquid by contracting the capacity; a suction section for suctioning liquid into the pump section; a discharge section for discharging liquid from the pump section; a suctioning-side check valve that suppresses a backflow of liquid from the pump section to the suction section; a discharge-side check valve that suppresses a backflow of liquid from the discharge section to the pump section; and a communication portion (6b, 60b) that provides continuous communication between the pump section and the discharge section in a state in which the discharge-side check valve suppresses the backflow, wherein a size of the communication portion is set so that, when the continuously expanding and contracting pump section contracts, a volume of gas mixed in liquid in the pump section is contracted; the discharge-side check valve is arranged on an upper side of the suctioning-side check valve; characterised in that the communication portion is provided on the discharge side check valve; and in that the pump section is provided with: a first channel having a fixed capacity with an upper end portion connected to the discharge-side check valve and a lower end portion connected to the suctioning-side check valve; and a second channel having a fixed capacity with one end portion connected to the first channel and the other end portion connected to the pump chamber. a first channel having a fixed capacity with an upper end portion connected to the discharge-side check valve and a lower end portion connected to the suctioning-side check valve; and a second channel having a fixed capacity with one end portion connected to the first channel and the other end portion connected to the pump chamber.