A filter state estimation system is configured to estimate a state of a filter provided in a hydraulic circuit. The hydraulic circuit includes: the filter; a bypass channel circumventing the filter and through which an oil flows; and a valve provided in the bypass channel and configured to be opened and closed based on a differential pressure. The filter state estimation system includes: a sensor to detect that the valve is in an open state or in a closed state; a temperature sensor to detect a temperature of the oil; and a state estimating unit to estimate a state of the filter based on detection results detected by the sensor and the temperature sensor. The state estimating unit estimates the state of the filter based on the temperature of the oil which is detected by the temperature sensor when the sensor detects that the valve is in the open state.