PROBLEM TO BE SOLVED: To provide a cooling water suction prevention cap which further improves suction of cooling water into an upper lid.SOLUTION: There is provided a cooling water suction prevention cap 1 which comprises: a cap body 3; and an upper lid 2 hinge-connected to the cap body, wherein the cap body 3 includes: a top wall 10; an inner cylinder vertically provided on a lower surface 10c of the top wall 10; a sealing cylinder 11 continuing to an outer peripheral edge part 10a of the top wall 10; an outer cylinder 9 continuing to a lower part 11b of the sealing cylinder 11; and an engaging cylinder 12 erected on a lower end part 11a of the sealing cylinder 11, a fitting groove 20 is formed between the inner cylinder 14 and the sealing cylinder 11, and the upper lid 2 has a ceiling wall 5 and a skirt wall 6 continuing to an outer peripheral edge part 5a of the ceiling wall 5. The skirt wall 6 is pressure contacted to the engaging cylinder 12 and the sealing cylinder 11 when the upper lid is closed and a
