A paper sheet humidifying device including a humidifying roller to convey a paper sheet and apply water to the paper sheet; a water supply roller to supply water to the humidifying roller; a water supply plate to contain the water for dipping the water supply roller, and to eject excessive water by overflowing; a water feeding section to feed the water to the water supply plate; a drain tank to receive the water ejected from the water supply plate, the drain tank comprising a drain outlet to eject the received water; a water level sensor to output a signal when a water level in the drain tank reaches a prescribed level; and a controller to execute a control to prevent overflowing of the water from the drain tank based on the signal from the water level sensor.