Disclosed is a pneumatic tire adopting a bias ply structure in which plural carcass layers are disposed so that cords cross each other between the layers, and having an aspect ratio of 55% or less and an outer diameter of 300 mm or less, wherein a cord angle to a tire circumferential direction of said carcass layer is set to a range from 27 to 37° in the center line position, and a surface shape of a tread portion is formed to be straight to a tire width direction in the region of 60 to 75% of a tread width.