To provide a snap fastener in which a male part and a female part can be easily engaged and fixed, and also they can be released from the fixation thereof only under a given condition.

A seal part is formed by pressedly inserting a rivet stem 9 into insertion holes, and by holding the inner peripheries of the respective bored parts of sheet members 3. A male part 13 is provided with an engaging projection 19 projected outside in the radial direction at the end of its cylindrical engaging part 14, and the male part locking part 20 of a female part 18 is provided with an elastic locking member 22 that has been arranged so as to be displaced outward in the radial direction with regard to the engaging part 14. The elastic locking member 22 has at least one lock-projection piece 23 projected inward in the radial direction, and this locks the engaging projection at the time of engagement of the male part 13 with the female part 18 for restraining the movement along its axis.