An inspection method for inspecting an insulator for a spark plug, the insulator having an axial hole formed therethrough in a direction of an axis of the insulator and including a collar portion, a rear body portion opposite to the spark discharge side located in rear of the collar portion and made smaller in diameter than the collar portion and a leg portion located in front of the collar portion and made smaller in diameter than the collar portion, the inspection method comprising: setting a certain point in the direction of the axis on the leg portion as a first measurement point and setting a certain point in the direction of the axis on any portion of the insulator other than the leg portion as a second measurement point; performing measurement of a distance in a radial direction of the insulator between an outer circumferential surface of the leg portion at the first measurement point and an outer circumferential surface of the any portion of the insulator other than the leg portion at the second measurement point by taking an image of the leg portion and the any portion of the insulator other than the leg portion with no contact to the leg portion and the any portion of the insulator other than the leg portion; repeating the setting of the first and second measurement points and the measurement of the distance at a plurality of positions in a circumferential direction of the insulator by causing rotation of the insulator about the axis in a state that at least one of the collar portion and the rear body portion of the insulator is held with a holding member, and thereby changing the position of the measurement in accordance with the rotation of the insulator; determining, as a circular runout (Dmax - Dmin), a difference between maximum and minimum values among results of the measurement of the distance at the plurality of positions; and making pass/fail judgment of the insulator based on the determined circular runout (Dmax - Dmin). setting a certain point in the direction of the axis on the leg portion as a first measurement point and setting a certain point in the direction of the axis on any portion of the insulator other than the leg portion as a second measurement point; performing measurement of a distance in a radial direction of the insulator between an outer circumferential surface of the leg portion at the first measurement point and an outer circumferential surface of the any portion of the insulator other than the leg portion at the second measurement point by taking an image of the leg portion and the any portion of the insulator other than the leg portion with no contact to the leg portion and the any portion of the insulator other than the leg portion; repeating the setting of the first and second measurement points and the measurement of the distance at a plurality of positions in a circumferential direction of the insulator by causing rotation of the insulator about the axis in a state that at least one of the collar portion and the rear body portion of the insulator is held with a holding member, and thereby changing the position of the measurement in accordance with the rotation of the insulator; determining, as a circular runout (Dmax - Dmin), a difference between maximum and minimum values among results of the measurement of the distance at the plurality of positions; and making pass/fail judgment of the insulator based on the determined circular runout (Dmax - Dmin).