A spark plug includes a tubular housing, a tubular insulator retained in the housing, a center electrode secured in the insulator, and a ground electrode provided at a distal end of the housing. The housing has a seat portion formed on its inner periphery. The insulator has a distal portion, a proximal portion, and a shoulder formed on an outer periphery of the insulator between the distal and proximal portions. The shoulder is arranged to seat on the seat portion of the housing with an annular packing interposed therebetween. On an inner peripheral surface of the seat portion of the housing which faces an outer peripheral surface of the distal portion of the shoulder, there are formed uneven portions that are arranged in a circumferential direction of the spark plug. Each of the uneven portions consists of a protrusion and a recess that adjoin each other in the circumferential direction.