A temperature sensor comprising: a cylindrical metal tube extending in an axial direction and having a front end side blocked; a thermal sensing element held in an inside of said metal tube and including a thermal sensing portion with electrical characteristic varying according to a temperature, and a pair of electrode wires provided in said thermal sensing portion and extending toward a rear end side of said metal tube; an electrically insulating member, wherein a heat conductivity of said electrically insulating member is not lower than 1.2 W/m·K; and a sheath member held in an inside of said metal tube and including: a sheath pipe, a pair of metal cores held in the sheath pipe and connected to said pair of electrode wires of said thermal sensing element while electrically insulated, and electrically insulating powder filled in between the sheath pipe and each of the metal cores, wherein: said metal tube has a small-diameter portion located on a front end side and entirely having an inner diameter smaller than an outer diameter of said sheath member, and a large-diameter portion (32, 34, 36) located on a rear end side of said small-diameter portion and having a diameter larger than an outer diameter of said small-diameter portion; an outer diameter of said small-diameter portion is not larger than 3.5 mm; a shortest distance L between said thermal sensing portion and an inner wall surface of the small-diameter portion satisfies 0 ≤ L ≤ 0.3 mm; and said thermal sensing portion is held in said small-diameter portion, characterized in that a longest distance H between a front end of said thermal sensing portion and a front end of an inner wall of said metal tube is not larger than 2.0 mm, and in that the electrically insulating member is filled at least in between a front end of said thermal sensing portion and a front end of an inner wall of said metal tube, so that the front end of the thermal sensing portion and the front end of the inner wall of the metal tube are in contact with each other through the electrically insulating member. a cylindrical metal tube extending in an axial direction and having a front end side blocked; a thermal sensing element held in an inside of said metal tube and including a thermal sensing portion with electrical characteristic varying according to a temperature, and a pair of electrode wires provided in said thermal sensing portion and extending toward a rear end side of said metal tube; an electrically insulating member, wherein a heat conductivity of said electrically insulating member is not lower than 1.2 W/m·K; and a sheath member held in an inside of said metal tube and including: a sheath pipe, a pair of metal cores held in the sheath pipe and connected to said pair of electrode wires of said thermal sensing element while electrically insulated, and electrically insulating powder filled in between the sheath pipe and each of the metal cores, wherein: said metal tube has a small-diameter portion located on a front end side and entirely having an inner diameter smaller than an outer diameter of said sheath member, and a large-diameter portion (32, 34, 36) located on a rear end side of said small-diameter portion and having a diameter larger than an outer diameter of said small-diameter portion; an outer diameter of said small-diameter portion is not larger than 3.5 mm; a shortest distance L between said thermal sensing portion and an inner wall surface of the small-diameter portion satisfies 0 ≤ L ≤ 0.3 mm; and said thermal sensing portion is held in said small-diameter portion, characterized in that a longest distance H between a front end of said thermal sensing portion and a front end of an inner wall of said metal tube is not larger than 2.0 mm, and in that the electrically insulating member is filled at least in between a front end of said thermal sensing portion and a front end of an inner wall of said metal tube, so that the front end of the thermal sensing portion and the front end of the inner wall of the metal tube are in contact with each other through the electrically insulating member. said metal tube has a small-diameter portion located on a front end side and entirely having an inner diameter smaller than an outer diameter of said sheath member, and a large-diameter portion (32, 34, 36) located on a rear end side of said small-diameter portion and having a diameter larger than an outer diameter of said small-diameter portion; an outer diameter of said small-diameter portion is not larger than 3.5 mm; a shortest distance L between said thermal sensing portion and an inner wall surface of the small-diameter portion satisfies 0 ≤ L ≤ 0.3 mm; and said thermal sensing portion is held in said small-diameter portion, characterized in that a longest distance H between a front end of said thermal sensing portion and a front end of an inner wall of said metal tube is not larger than 2.0 mm, and in that the electrically insulating member is filled at least in between a front end of said thermal sensing portion and a front end of an inner wall of said metal tube, so that the front end of the thermal sensing portion and the front end of the inner wall of the metal tube are in contact with each other through the electrically insulating member.