A glow plug comprising: a heater having a heat generating resistor that generates heat by passing an electric current therethrough; a metal shell formed in a tubular shape having a shaft hole extending in an axial direction and which directly holds the heater in a front end part thereof or indirectly through a holding member; an intermediate shaft formed in the shape of a rod, arranged in the shaft hole of the metal shell with a clearance provided relative to an inner peripheral surface of the metal shell and which has one end part thereof connected to a rear end part of the heater and the other end part thereof protruding from a rear end of the metal shell; and a cylindrical insulator arranged between the shaft hole of the metal shell and the intermediate shaft, which insulator is urged toward the front end of the metal shell in the axial direction and positioned by a fixing part provided in the other end part of the intermediate shaft, further including: a first tapered part that is provided in the inner peripheral surface of the metal shell in a rear end part of the metal shell and which expands from the shaft hole to an opening of the shaft hole; and a second tapered part that is provided in an outer peripheral surface of the insulator, expands from an end side to a rear end side of the insulator and abuts the first tapered part, characterized in that the glow plug further comprising: a rear end surface of the insulator abuts an end surface of the fixing part, when a maximum outside diameter of the first tapered part of the metal shell is D1, a maximum outside diameter of the second tapered part of the insulator is D2, a maximum outside diameter of the rear end surface of the insulator is D3 and a maximum outside diameter of the end surface of the fixing part is D4, wherein the relations of D1 > D2 and D3 < D4 are satisfied. a heater having a heat generating resistor that generates heat by passing an electric current therethrough; a metal shell formed in a tubular shape having a shaft hole extending in an axial direction and which directly holds the heater in a front end part thereof or indirectly through a holding member; an intermediate shaft formed in the shape of a rod, arranged in the shaft hole of the metal shell with a clearance provided relative to an inner peripheral surface of the metal shell and which has one end part thereof connected to a rear end part of the heater and the other end part thereof protruding from a rear end of the metal shell; and a cylindrical insulator arranged between the shaft hole of the metal shell and the intermediate shaft, which insulator is urged toward the front end of the metal shell in the axial direction and positioned by a fixing part provided in the other end part of the intermediate shaft, further including: a first tapered part that is provided in the inner peripheral surface of the metal shell in a rear end part of the metal shell and which expands from the shaft hole to an opening of the shaft hole; and a second tapered part that is provided in an outer peripheral surface of the insulator, expands from an end side to a rear end side of the insulator and abuts the first tapered part, characterized in that the glow plug further comprising: a rear end surface of the insulator abuts an end surface of the fixing part, when a maximum outside diameter of the first tapered part of the metal shell is D1, a maximum outside diameter of the second tapered part of the insulator is D2, a maximum outside diameter of the rear end surface of the insulator is D3 and a maximum outside diameter of the end surface of the fixing part is D4, wherein the relations of D1 > D2 and D3 < D4 are satisfied. a first tapered part that is provided in the inner peripheral surface of the metal shell in a rear end part of the metal shell and which expands from the shaft hole to an opening of the shaft hole; and a second tapered part that is provided in an outer peripheral surface of the insulator, expands from an end side to a rear end side of the insulator and abuts the first tapered part, characterized in that the glow plug further comprising: a rear end surface of the insulator abuts an end surface of the fixing part, when a maximum outside diameter of the first tapered part of the metal shell is D1, a maximum outside diameter of the second tapered part of the insulator is D2, a maximum outside diameter of the rear end surface of the insulator is D3 and a maximum outside diameter of the end surface of the fixing part is D4, wherein the relations of D1 > D2 and D3 < D4 are satisfied.