A ground electrode of an igniter plug has inlets for supplying cooling fluid therethrough to a first space formed between an insulator and the ground electrode, and first outlets located forward of the inlets and radially outward of the inner circumference of a ground electrode forward-end portion and adapted to discharge the cooling fluid therethrough. A second space communicating with the first space and having a second outlet for discharging the cooling fluid therethrough is formed between a forward end surface of the insulator and a surface of the ground electrode which faces the forward end surface.