Disclosed is a breather system for a fuel tank reliably performing a function of a breather when a vehicle is inclined to the right or to the left and a straddle-type four wheeled all terrain vehicle comprising the breather system in which the fuel tank is provided below a seat, and a fuel inlet and a tank cap covering the fuel inlet is protruded from a rear fender laterally of the seat. The breather system has two breather openings provided in the fuel tank, breather hoses respectively connected to the breather openings, a T-shape joint integrally connecting the breather hoses, and a check valve connected to an ambient side of the T-shape joint, and the two breather openings are provided on the right and left sides of the fuel tank so as to be spaced apart from each other.