A small-sized spark plug for internal combustion engines is disclosed which is composed of a center electrode having a shaft portion surrounded by an insulator layer and a terminal end portion adapted to act as a spark generation portion; and an earthed electrode having a terminal end portion which is spaced apart from and is opposite to the terminal end portion of the center electrode so that a spark gap is defined along a surface of the insulator layer. The shortest distance .delta. (mm) between each end portion of the two electrodes and the surface of the insulation layer and the distance l between the forward end portions of the two electrodes have the following relationship:.delta..gtoreq.0.25 l, and 0.8 mm.ltoreq.l.ltoreq.2.0 mm.A recessed portion may be provided on the surface of the insulation layer.