According to one embodiment, a substrate processing apparatus includes a tank that stores a treatment liquid; a liquid level pipe connected to the tank such that the treatment liquid stored in the tank flows therein, and configured such that the liquid level of the treatment liquid therein moves according to increase and decrease of the treatment liquid in the tank; a liquid level sensor that detects the liquid level in the liquid level pipe; an air supply pipe for supplying a gas to a piping space above the liquid level in the liquid level pipe; and a controller that determines whether there is erroneous detection of the liquid level sensor based on a detection result obtained by the liquid level sensor in response to the movement of the liquid level in the liquid level pipe caused by supply of the gas to the piping space from the air supply pipe.