Ultrasonic cleaning method

A cleaning method includes the steps of preparing a cleaning liquid (S10), and cleaning an object to be cleaned by dipping the object to be cleaned in the cleaning liquid while irradiating the cleaning liquid ultrasonic waves (S20). In the cleaning step (S20), the dissolved nitrogen concentration in the cleaning liquid has been adjusted in accordance with the size of a foreign substance that is to be removed from the object to be cleaned with the highest removal efficiency. In this way, a foreign substance of a specific size can be efficiently removed by changing the dissolved nitrogen concentration in the cleaning liquid in accordance with the size of the foreign substance to be removed.