A glow plug comprising: a sheath tube having a side portion extending in an axial direction (OD) and a forward end portion connected to a forward end of the side portion and closing the forward end of the side portion , and a heat generation coil disposed in the sheath tube in such a manner as to extend in the axial direction (OD) and whose forward end portion is connected to the forward end portion of the sheath tube , wherein the forward end portion of the heat generation coil is surrounded by and embedded in the forward end portion of the sheath tube ; characterized by : an alloy portion located between the heat generation coil and the forward end portion of the sheath tube and formed of an alloy of a metal used to form the sheath tube and a metal used to form the heat generation coil has a thickness of 10 μm or less; and by a minimum thickness A of the side portion of the sheath tube as measured in a region along the axial direction (OD) where the heat generation coil is disposed, a distance B along the axial direction (OD) from a forward end of the sheath tube to a forward end of the heat generation coil , and a maximum thickness C of the forward end portion of the sheath tube along the axial direction (OD) satisfying the relational expressions: B > A and C / A ≤ 2.5 . a sheath tube having a side portion extending in an axial direction (OD) and a forward end portion connected to a forward end of the side portion and closing the forward end of the side portion , and a heat generation coil disposed in the sheath tube in such a manner as to extend in the axial direction (OD) and whose forward end portion is connected to the forward end portion of the sheath tube , wherein the forward end portion of the heat generation coil is surrounded by and embedded in the forward end portion of the sheath tube ; characterized by : an alloy portion located between the heat generation coil and the forward end portion of the sheath tube and formed of an alloy of a metal used to form the sheath tube and a metal used to form the heat generation coil has a thickness of 10 μm or less; and by a minimum thickness A of the side portion of the sheath tube as measured in a region along the axial direction (OD) where the heat generation coil is disposed, a distance B along the axial direction (OD) from a forward end of the sheath tube to a forward end of the heat generation coil , and a maximum thickness C of the forward end portion of the sheath tube along the axial direction (OD) satisfying the relational expressions: B > A and C / A ≤ 2.5 . an alloy portion located between the heat generation coil and the forward end portion of the sheath tube and formed of an alloy of a metal used to form the sheath tube and a metal used to form the heat generation coil has a thickness of 10 μm or less; and by a minimum thickness A of the side portion of the sheath tube as measured in a region along the axial direction (OD) where the heat generation coil is disposed, a distance B along the axial direction (OD) from a forward end of the sheath tube to a forward end of the heat generation coil , and a maximum thickness C of the forward end portion of the sheath tube along the axial direction (OD) satisfying the relational expressions: B > A and C / A ≤ 2.5 . <EOS>