A valve abnormality detecting device includes an opening acquiring portion to acquire a valve opening value; a pressure acquiring portion to acquire a pressure value of operating device air of an operating device for a valve; a stability-time detecting portion configured to detect a stable-opening state in which the valve opening value acquired by the opening acquiring portion 1 is substantially constant; a frictional force detecting portion configured to detect a difference between a maximum pressure value and a minimum pressure value of the operating device air in the stable-opening state as an index value indicating a frictional force at a sliding portion of the valve; and an abnormality determining portion configured to determine that an abnormality may have occurred in the valve in a case where a frequency of occurrence of reduction in which the index value falls below a specified value is an abnormal frequency.