An engine starter apparatus comprises a housing (9), a d.c. electric motor (2), an output rotary shaft (3) with a pinion (7), and a planetary speed reduction gear (5). The planetary speed reduction gear (5) comprises a sun gear (11), an internal gear (20), a plurality of planetary gears (12) disposed between the sun gear (11) and the internal gear (20), and a planetary gear carrier (10) connected to the output rotary shaft (3) for rotatably supporting the planetary gears (12). The internal gear (20) comprises a ring-shaped internal gear member (20a), and elastic reinforcing ring member (21,22; 24) for elastically supporting the internal gear member (20a), thereby to increase rigidity of the internal gear member (20a). The reinforcing ring member may comprise a thin-wall cylindrical member (22) concentrically disposed around the internal gear member (20a) with an annulus defined therebetween, and a plurality of ribs (21) radially and axially extending through the annulus for integrally connecting them together. The reinforcing ring member may be a solid elastic ring (24). The reinforcing ring member (21,22; 24) may have formed therein an axial groove (23) for allowing a through bolt for connecting the housing and the d.c. motor to pass therethrough.