A glow plug wherein a combustion gas prevention wall (67) extends radially outward on the outer circumferential surface of heater (10) such that when the heater (10) is viewed rearward from its forward end (10a) along the axial direction, the combustion gas prevention wall (67) closes an annular gap K1 at a position forward of annular membrane portion (63) of seal member (60). The combustion gas prevention wall (67) has an outer diameter D2 greater than the inner diameter D1 of forward end (136) of the housing.