A glow plug including a substantially cylindrical housing extending in an axial direction; a rod-shaped heater unit having a front end protruding from the front end of the housing and movable in the axial direction; a connecting member connecting the heater unit to the housing within the housing and allowing the heater unit to move in the axial direction; and a pressure sensor that detects pressure received by the heater unit. The heater unit has a large-diameter portion at the rear end thereof and a small-diameter portion frontward of the large-diameter portion and having a diameter smaller than the diameter of the large-diameter portion. The connecting member connects the small-diameter portion of the heater unit to the housing within the housing.