To provide a synthetic resin cap free from a variation in opening strength, highly sealable, and preferably opened.

The synthetic resin cap 1 includes a top wall 2 and a skirt wall 3 extending downward from the circumferential edge of the top wall, the skirt wall 3 having an opening flange 4 provided in the area of part of the periphery, and engaging projections provided at the lower end of the internal circumferential surface along its entire circumference at spaced intervals. The skirt wall 3 has weakened portions 8 extending upward from the lower end. The weakened portions 8 are provided so as to be continuous circumferentially with the engaging projections.