An ignition plug (1) includes a center electrode (5) extending in the direction of an axis (CL1), an insulator (2) having an axial hole (4) into which the center electrode (5) is inserted, a tip (31) joined to a forward end portion of the center electrode (5) by a fusion portion (35), and a ground electrode (27) forming a gap (33) in cooperation with the tip (31). The ignition plug generates high-frequency plasma at the gap (33) when high-frequency electric power is supplied to the gap (33). The forward end of the tip (31) is located forward of the forward end of the insulator (2) with respect to the direction of the axis (CL1). Also, at least a portion of the outer surface of the fusion portion (35) is located within the axial hole (4), and, the distance between a forward-end-side opening of the axial hole (4) and the rearmost end of the outer surface of the fusion portion (35), measured along the axis (CL1), is equal to or greater than 0.1 mm. Thus, in the ignition plug (1) which generates high-frequency plasma, corrosion of the fusion portion can be suppressed effectively, whereby durability is improved.