An ignition plug includes a tubular ceramic insulator and a metallic shell having a protrusion protruding radially inward. The ceramic insulator has an engagement portion which is engaged with a receiving surface of the protrusion and an intermediate trunk portion extending rearward from the rear end of the engagement portion. A≦1.70 and B ≧1.20 are satisfied, where A is the thickness (mm) of the metallic shell along a direction which passes through the center of the receiving surface and is orthogonal to the axial line on a cross section including the axial line and B is the minimum thickness (mm) of the metallic shell 3 at the tube portion along the direction orthogonal to the axial line.