An ignition plug having enhanced seal performance, while suppressing deformation of a metallic shell thereof. The ignition plug includes a metallic shell having a crimp portion, a male screw portion, a seat portion, and a bent portion. The bent portion has a largest outer diameter (OD) portion, and first and second smallest OD portions, wherein 0.01≦(L2+L3−L1)/(L2+L3)≦0.3 is satisfied. L1 is distance between a point in the first smallest OD portion farthest from center axis and a point in the second smallest OD portion farthest from center axis, L2 is distance between the point in the first smallest OD portion farthest from center axis and a point in the largest OD portion farthest from center axis, and L3 is distance between the point in the second smallest OD portion farthest from center axis and the point in the largest OD portion farthest from center axis.