To provide a cap having an annular slit adequately formed therein.

The cap is equipped with a cylindrical part 11, a top part 12 provided so as to cover an upper edge of the cylindrical part 11, and an annular part 13 provided on a lower portion of the cylindrical part 11. An annular slit 15 for cutting the annular part 13 into upper and lower portions. An annular inner ring 14 extends to a lower face 12a of the top part 12 with a predetermined distance L from an inner surface 11a of the cylindrical part 11. A cap C closes a spout 2 by tightening a tip 3T of the cylindrical spout 21 between the inner surface 11a of the cylindrical part 11 and the inner ring 14. A recess 12c is formed in an upper face 12b of the top part 12. The horizontal projected area π[(X2)/2]2 of the recess 12c is smaller than the horizontal projected area π[(X1)/2]2 inside the inner ring 14.