An object is to provide a filler cap of a fuel tank capable of securely preventing a disadvantage that, when the fuel tank tilts, fuel stored in the fuel tank is discharged from an air passage disposed at a cap main body. The cap includes the air passage which is constituted at the cap main body and which allows the inside of the fuel tank to communicate with the outside, and a valve mechanism portion constituted at this air passage. The valve mechanism portion includes a tube main body having a tube-like portion and a reduced diameter portion formed externally and continuously from this tube-like portion, a float stored in the tube main body with a space between the float and an inner surface of this tube main body, a piston positioned on a fuel tank side of the float and stored in the tube main body with a space between the piston and the inner surface of the tube main body, and a spherical member positioned on an outer side of the float and stored in the tube main body.