A protective member forming method for forming a protective member on a surface on one side of a wafer includes a resin state recognizing step of heating a thermoplastic resin in contact with the surface on the one side of the wafer while propagating an ultrasonic vibration between a resin placing surface of a stage and a wafer holding surface and recognizing whether or not the thermoplastic resin sandwiched between the surface on the one side of the wafer held on the wafer holding surface and the resin placing surface has been unified. When the thermoplastic resin is recognized to have been unified, the thermoplastic resin is spread out onto a whole area of the surface on the one side of the wafer, followed by cooling the thermoplastic resin to cure the thermoplastic resin.