A heater section includes a plate-like ceramic body (a first substrate layer 1, a second substrate layer 2, and a third substrate layer 3) having a longitudinal direction (front-rear direction) and a short-length direction (left-right direction), and a heater 72 disposed within the plate-like ceramic body and including a lead section 79 and a heating section 76 connected to the lead section 79. The heating section 76 includes a straight portion 78 extending along the longitudinal direction, and a lead side curved portion 77b connected to one of the ends of the straight portion 78 closer to the lead section 79. The lead side curved portion 77b has a lower resistance per unit length than the straight portion 78 at one or more temperatures in the range of 700° C. to 900° C.