A method of manufacturing a spark plug includes the step of adjusting a spark gap in the spark plug. In the step, a ground electrode is repeatedly pressed, by a hammer, toward a center electrode. The hammer operates in a first mode when the size of the spark gap falls in a rough-process range which is above a predetermined value, and in a second mode when the size of the spark gap falls in a finish-process range which is between the predetermined value and a target value less than the predetermined value. The amount of pressing the ground electrode in any press stroke of the hammer in the second mode is less than that in any press stroke of the hammer in the first mode. The amount of pressing the ground electrode in every press stroke of the hammer in the second mode is equal to a fixed value.