A high-frequency wave supplying structure includes a center conductor that extends in a specified direction, an outer conductor that is coaxial with the center conductor and grounded, and a cylindrical insulating member that is provided between the center conductor and the outer conductor. The distal end of the center conductor is a support portion that supports a workpiece W. A shield member is provided outward of the outer conductor to be coaxial with the outer conductor and the center conductor. The distal end of the shield member is located closer to the support portion in the specified direction than the distal end of the outer conductor. The insulating member includes a protruding portion that protrudes toward the support portion from an opening of the outer conductor. The protruding portion is opposed to the distal end of the outer conductor in the specified direction.