A deterioration determination device comprising a liquid pumping apparatus, said liquid pumping apparatus comprising: a casing that forms a reservoir space for storing liquid flowed thereto; a feed valve that introduces working gas into the reservoir space; an exhaust valve that releases the working gas from the reservoir space; and a valve operating mechanism having a float arranged in the reservoir space, and performing a pumping stroke in which the liquid is pumped from the reservoir space under a pressure of the working gas by opening the feed valve and closing the exhaust valve when the float moves up to a predetermined high level, and an inflow stroke in which the liquid flows into the reservoir space and the working gas is released from the reservoir space by closing the feed valve and opening the exhaust valve when the float moves down to a predetermined low level, wherein the deterioration determination device comprising: a first pressure sensor arranged to communicate with a lower portion of the reservoir space to detect a pressure of a liquid layer in the reservoir space; and a deterioration determination unit that is configured to determine a degree of deterioration of the valve operating mechanism, characterized in that a second pressure sensor is arranged to communicate with an upper portion of the reservoir space to detect a pressure of a gaseous layer in the reservoir space, wherein the deterioration determination unit is configured to determine the degree of deterioration of the valve operating mechanism based on a differential pressure obtained by subtracting the pressure detected by the second pressure sensor from the pressure detected by the first pressure sensor, wherein the deterioration determination unit determines that the valve operating mechanism is deteriorated when at least one of conditions is met, the conditions including a condition that the differential pressure is larger than a differential pressure at the time when the inflow stroke is normally switched to the pumping stroke, and a condition that the differential pressure is smaller than a differential pressure at the time when the pumping stroke is normally switched to the inflow stroke. a casing that forms a reservoir space for storing liquid flowed thereto; a feed valve that introduces working gas into the reservoir space; an exhaust valve that releases the working gas from the reservoir space; and a valve operating mechanism having a float arranged in the reservoir space, and performing a pumping stroke in which the liquid is pumped from the reservoir space under a pressure of the working gas by opening the feed valve and closing the exhaust valve when the float moves up to a predetermined high level, and an inflow stroke in which the liquid flows into the reservoir space and the working gas is released from the reservoir space by closing the feed valve and opening the exhaust valve when the float moves down to a predetermined low level, wherein the deterioration determination device comprising: a first pressure sensor arranged to communicate with a lower portion of the reservoir space to detect a pressure of a liquid layer in the reservoir space; and a deterioration determination unit that is configured to determine a degree of deterioration of the valve operating mechanism, characterized in that a second pressure sensor is arranged to communicate with an upper portion of the reservoir space to detect a pressure of a gaseous layer in the reservoir space, wherein the deterioration determination unit is configured to determine the degree of deterioration of the valve operating mechanism based on a differential pressure obtained by subtracting the pressure detected by the second pressure sensor from the pressure detected by the first pressure sensor, wherein the deterioration determination unit determines that the valve operating mechanism is deteriorated when at least one of conditions is met, the conditions including a condition that the differential pressure is larger than a differential pressure at the time when the inflow stroke is normally switched to the pumping stroke, and a condition that the differential pressure is smaller than a differential pressure at the time when the pumping stroke is normally switched to the inflow stroke.