Filler cap for fuel tank which can further prevent environmental pollution without releasing harmful gas evaporated from fuel in fuel tank and can improve fuel efficiency is provided. When vaporized gas is generated in fuel tank, inner pressure of fuel tank is increased to pressure for deformation strength prescribed to valve body or more, a central part of valve body enters into third space thereby causing deformation so to shorten outer diameter of valve body to form wrinkles, and in peripheral part of valve body are formed a portion contacting to pivot of cylinder main body when deforming and concave portion not contacting thereto, second space and third space are connected via space of this concave portion and air passage channel formed in wall surface forming second space to release excessive pressure to outside fuel tank thereby bringing pressure inside fuel tank to lower than the pressure for deformation strength.