A medical liquid-pressure-detecting device comprising: a round, spherical chamber unit connected to a liquid flow route and having an inlet from which liquid flowing in the flow route is taken in and an outlet from which the liquid is discharged, the chamber unit being capable of storing the liquid by a predetermined amount while the liquid flows from the inlet to the outlet; and a diaphragm dividing an inside of the chamber unit into a liquid chamber configured to store the liquid and a gas chamber configured to store a predetermined gas, the diaphragm being deformable in accordance with a pressure of the liquid in the liquid chamber, the medical liquid-pressure-detecting device being capable of detecting a pressure of the liquid in the flow route on the basis of a change in the pressure in the gas chamber that is caused by the deformation of the diaphragm, wherein the diaphragm includes a first region configured to deform toward the liquid chamber when a negative pressure is generated in the liquid stored in the liquid chamber and which is referred to as negative-pressure-detecting region, and a second region configured to deform toward the gas chamber when a positive pressure is generated in the liquid stored in the liquid chamber and which is referred to as positive-pressure-detecting region, wherein the negative-pressure-detecting region forms an arc shape projection downward between the peripheral edge of the diaphragm and the positive-pressure-detecting region, wherein the positive-pressure-detecting region forms an arc shape projection upwards at the center of the diaphragm wherein the positive-pressure-detecting region and the negative-pressure-detecting region are gently continuous with each other from the center toward a peripheral edge of the diaphragm and wherein the positive-pressure-detecting region is situated at the center, wherein the outlet is provided at a top of the liquid chamber of the chamber unit and at a center position in a widthwise direction, and the inlet is provided at a position of the liquid chamber that is lower and thereby nearer to the diaphragm than the outlet and is shifted from a center position in the widthwise direction by a predetermined length and the inlet is directed off-center of the liquid chamber wherein the position of the inlet is adapted that the liquid introduced from the inlet flows upward in a direction away from the diaphragm while forming a swirl and wherein the positive-pressure-detecting region is a region projecting upward in direction of the outlet at a center of the diaphragm and is positioned below a center of a swirl of the liquid that is generated when the liquid flows from the inlet to the outlet. a round, spherical chamber unit connected to a liquid flow route and having an inlet from which liquid flowing in the flow route is taken in and an outlet from which the liquid is discharged, the chamber unit being capable of storing the liquid by a predetermined amount while the liquid flows from the inlet to the outlet; and a diaphragm dividing an inside of the chamber unit into a liquid chamber configured to store the liquid and a gas chamber configured to store a predetermined gas, the diaphragm being deformable in accordance with a pressure of the liquid in the liquid chamber, the medical liquid-pressure-detecting device being capable of detecting a pressure of the liquid in the flow route on the basis of a change in the pressure in the gas chamber that is caused by the deformation of the diaphragm, wherein the diaphragm includes a first region configured to deform toward the liquid chamber when a negative pressure is generated in the liquid stored in the liquid chamber and which is referred to as negative-pressure-detecting region, and a second region configured to deform toward the gas chamber when a positive pressure is generated in the liquid stored in the liquid chamber and which is referred to as positive-pressure-detecting region, wherein the negative-pressure-detecting region forms an arc shape projection downward between the peripheral edge of the diaphragm and the positive-pressure-detecting region, wherein the positive-pressure-detecting region forms an arc shape projection upwards at the center of the diaphragm wherein the positive-pressure-detecting region and the negative-pressure-detecting region are gently continuous with each other from the center toward a peripheral edge of the diaphragm and wherein the positive-pressure-detecting region is situated at the center, wherein the outlet is provided at a top of the liquid chamber of the chamber unit and at a center position in a widthwise direction, and the inlet is provided at a position of the liquid chamber that is lower and thereby nearer to the diaphragm than the outlet and is shifted from a center position in the widthwise direction by a predetermined length and the inlet is directed off-center of the liquid chamber wherein the position of the inlet is adapted that the liquid introduced from the inlet flows upward in a direction away from the diaphragm while forming a swirl and wherein the positive-pressure-detecting region is a region projecting upward in direction of the outlet at a center of the diaphragm and is positioned below a center of a swirl of the liquid that is generated when the liquid flows from the inlet to the outlet.