A protector (160) is inserted into a metallic shell (110) such that at least a portion of each gas introduction hole (167) is located on the base end side with respect to the forward end (110b) of the metallic shell. The metallic shell has a gas introduction space S2 defined by a forward-end-side inner surface (113b) of the metallic shell and an outer surface (160d) of the protector and which guides a gas-to-be-detected (exhaust gas G) from a region on the forward end side of the metallic shell into the gas introduction holes of the protector. The base end of the forward end portion (121) of the detection element (120) is located on the base end side with respect to the forward end (167b) of each gas introduction hole, and the forward end of the forward end portion is located on the forward end side with respect to the base end (167c) of each gas introduction hole.