A cap metal forming method capable of obtaining a uniform film thickness on the entire surface of a substrate is provided. The method for forming a cap metal on a copper wiring formed on a processing target surface of a substrate includes: holding the substrate so as to be rotatable; rotating the substrate in a processing target surface direction of the substrate; locating an end portion of an agitation member so as to face the processing target surface of a periphery portion of the substrate with a preset gap maintained therebetween; supplying a plating processing solution onto the processing target surface; and stopping the supply of the plating processing solution and moving the agitation member such that the end portion of the agitation member is separated away from the processing target surface of the substrate.