A glow plug (10, 10a) comprising: a sheath tube having a side portion extending in an axial direction 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 (820, 820a) disposed in the sheath tube in such a manner as to extend in the axial direction and whose forward end portion (822, 822a) is connected to the forward end portion of the sheath tube, wherein the forward end portion (822, 822a) of the heat generation coil (820, 820a) 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 (820, 820a) 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 (820, 820a) 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 where the heat generation coil (820, 820a) is disposed, a distance B along the axial direction from a forward end of the sheath tube to a forward end (821, 821a) of the heat generation coil (820, 820a), and a maximum thickness C of the forward end portion of the sheath tube along the axial direction satisfying the relational expressions: B > A and C / A ≤ 2.5. a sheath tube having a side portion extending in an axial direction 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 (820, 820a) disposed in the sheath tube in such a manner as to extend in the axial direction and whose forward end portion (822, 822a) is connected to the forward end portion of the sheath tube, wherein the forward end portion (822, 822a) of the heat generation coil (820, 820a) 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 (820, 820a) 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 (820, 820a) 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 where the heat generation coil (820, 820a) is disposed, a distance B along the axial direction from a forward end of the sheath tube to a forward end (821, 821a) of the heat generation coil (820, 820a), and a maximum thickness C of the forward end portion of the sheath tube along the axial direction satisfying the relational expressions: B > A and C / A ≤ 2.5. an alloy portion located between the heat generation coil (820, 820a) 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 (820, 820a) 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 where the heat generation coil (820, 820a) is disposed, a distance B along the axial direction from a forward end of the sheath tube to a forward end (821, 821a) of the heat generation coil (820, 820a), and a maximum thickness C of the forward end portion of the sheath tube along the axial direction satisfying the relational expressions: B > A and C / A ≤ 2.5.