A spark plug, comprising: a rod-shaped center electrode which extends in the direction of an axis; an insulator having a through hole which extends from a front end side to a rear end side in the direction of the axis and in which at least a portion of the center electrode is disposed in a front end side portion of the through hole; a metallic terminal at least a portion of which is disposed in a rear end side portion of the through hole and a rear end side portion of which is exposed out of the through hole; and a connecting portion (60, 70, 80) which connects electrically the center electrode to the metallic terminal in the through hole; wherein the insulator includes: a first portion which accommodates a front end of the metallic terminal and which has a first bore diameter of 2.9 mm or smaller; a second portion which is disposed closer to the rear end side than the first portion and which has a second bore diameter which is greater than the first bore diameter; and a middle portion which is disposed between the first portion and the second portion and a bore diameter of which increases towards the rear end side, wherein the metallic terminal includes a roughened surface portion formed at a position of the metallic terminal which corresponds to at least a part of the first portion, at least a part of the second portion and the middle portion of the insulator in the direction of the axis when the metallic terminal is inserted into the through hole in the insulator, the roughened surface having at least one of one or more projecting portions and one or more recess portions on an outer circumferential surface thereof, wherein a Vickers hardness of a portion of the metallic terminal which is disposed in the through hole and is disposed closer to the rear end side than the roughened surface portion is 200 Hv or greater and 320 Hv or smaller, and wherein a first ratio which is a ratio of an outside diameter of the roughened surface corresponding to the at least a part of the first portion of the insulator to the first bore diameter of the insulator is 0.90 or greater, characterized in that a second ratio which is a ratio of the first bore diameter to the second bore diameter of the insulator is 0.80 or greater and 0.98 or smaller. a rod-shaped center electrode which extends in the direction of an axis; an insulator having a through hole which extends from a front end side to a rear end side in the direction of the axis and in which at least a portion of the center electrode is disposed in a front end side portion of the through hole; a metallic terminal at least a portion of which is disposed in a rear end side portion of the through hole and a rear end side portion of which is exposed out of the through hole; and a connecting portion (60, 70, 80) which connects electrically the center electrode to the metallic terminal in the through hole; wherein the insulator includes: a first portion which accommodates a front end of the metallic terminal and which has a first bore diameter of 2.9 mm or smaller; a second portion which is disposed closer to the rear end side than the first portion and which has a second bore diameter which is greater than the first bore diameter; and a middle portion which is disposed between the first portion and the second portion and a bore diameter of which increases towards the rear end side, wherein a first portion which accommodates a front end of the metallic terminal and which has a first bore diameter of 2.9 mm or smaller; a second portion which is disposed closer to the rear end side than the first portion and which has a second bore diameter which is greater than the first bore diameter; and a middle portion which is disposed between the first portion and the second portion and a bore diameter of which increases towards the rear end side, wherein the metallic terminal includes a roughened surface portion formed at a position of the metallic terminal which corresponds to at least a part of the first portion, at least a part of the second portion and the middle portion of the insulator in the direction of the axis when the metallic terminal is inserted into the through hole in the insulator, the roughened surface having at least one of one or more projecting portions and one or more recess portions on an outer circumferential surface thereof, wherein a Vickers hardness of a portion of the metallic terminal which is disposed in the through hole and is disposed closer to the rear end side than the roughened surface portion is 200 Hv or greater and 320 Hv or smaller, and wherein a first ratio which is a ratio of an outside diameter of the roughened surface corresponding to the at least a part of the first portion of the insulator to the first bore diameter of the insulator is 0.90 or greater, characterized in that a second ratio which is a ratio of the first bore diameter to the second bore diameter of the insulator is 0.80 or greater and 0.98 or smaller.