A ceramic heater (1 - 12) comprising a substrate formed of an electrically insulative ceramic, and a resistor element buried in the substrate, wherein the resistor element includes a single heat-generating portion formed of an electrically conductive ceramic and folded into a U-like shape, and a pair of lead portions which are joined to opposite end portions of said heat-generating portion, the end portions facing rearward with respect to a direction of an axis XA, and which extend straight rearward with respect to the direction of the axis XA, wherein said resistor element includes intermediate portions located between said heat-generating portion and said lead portions; when, on cross sections S 1 and S 2 of said ceramic heater (1 - 12) taken along a plane perpendicular to said axis XA at a front end side point P 1 and a rear end side point P 2, which are arbitrary two different points on said axis XA, imaginary circumscribed circles CG 1 and CG 2 are drawn such that the imaginary circumscribed circles CG 1 and CG 2 circumscribe and contain two cross sections HS 1a and HS 1b and two cross sections HS 2a and HS 2b, respectively, of said resistor element appearing on the cross sections S 1 and S 2, respectively, diameter CL, and CL 2 of the circumscribed circles CG 1 and CG 2 satisfy a relation CL 1 < CL 2; and the total cross sectional area HS 1S of the two cross sections HS 1a and HS 1b of said resistor element and the total cross sectional area HS 2S of the two cross sections HS 2a and HS 2b of said resistor element satisfy a relation HS 1S < HS 2S; characterized in that said ceramic heater (1 - 12) is inserted into and held in a tubular member formed of metal such that a front end portion of said ceramic heater (1 - 12) is exposed; each of said intermediate portions has a portion having a thickness t XVex equal to or less than 2/3 a maximum thickness t XVmax of said resistor element; and a portion of said resistor element whose thickness is 2 (t XVmax)/3 is exposed from said tubular member formed of metal; said thickness being defined as the dimension of the resistor element in a direction perpendicular to the plane in which the U shape of said resistor element can be defined. said resistor element includes intermediate portions located between said heat-generating portion and said lead portions; when, on cross sections S 1 and S 2 of said ceramic heater (1 - 12) taken along a plane perpendicular to said axis XA at a front end side point P 1 and a rear end side point P 2, which are arbitrary two different points on said axis XA, imaginary circumscribed circles CG 1 and CG 2 are drawn such that the imaginary circumscribed circles CG 1 and CG 2 circumscribe and contain two cross sections HS 1a and HS 1b and two cross sections HS 2a and HS 2b, respectively, of said resistor element appearing on the cross sections S 1 and S 2, respectively, diameter CL, and CL 2 of the circumscribed circles CG 1 and CG 2 satisfy a relation CL 1 < CL 2; and the total cross sectional area HS 1S of the two cross sections HS 1a and HS 1b of said resistor element and the total cross sectional area HS 2S of the two cross sections HS 2a and HS 2b of said resistor element satisfy a relation HS 1S < HS 2S;