A substrate processing apparatus is provided that includes an ozonizer for generating ozone gas and an ozone sensor for detecting an ozone gas concentration. The substrate processing apparatus processes a substrate by using the ozone gas supplied from the ozonizer. The substrate processing apparatus includes a monitor unit for monitoring the ozone gas concentration detected by the ozone sensor and a control unit for detecting an abnormality of the ozone gas concentration based on the monitored ozone gas concentration and the monitored discharge power.