A seal member for a pneumatic tool is a flexible ring mounted to a ring-shaped cylinder in which a rotary piston is rotatably engaged. The cylinder has an inlet and an outlet radially defined therethrough. Two pairs of ribs respectively extend radially outward from the cylinder and the first inlet is located between one of the two pairs of ribs and the outlet inlet is located between the other pair of the two ribs. The seal member has a first hole communicating with the inlet and a second hole communicating with the outlet. The first hole is enclosed by a first flange and the second hole is enclosed by a second flange. One pair of the ribs is engaged with and enclosed by the first flanges the other pair of ribs is engaged with and enclosed by the second flange.