Abstract: Herein disclosed is a valve actuator which is to be used with a slide exhaust brake system. The valve actuator includes a cylinder casing which is constructed of: a shortened cyclinder body having a length substantially equal to the sliding distance of a piston member fitted therein; and a cylindrical wall fixed on the inner side of the bottom wall of the cylinder body such that the cylinder body has its lower end merging around its circumference into the circumference of the upper end of the cylindrical wall. The heat to be transferred from the exhaust brake system to the actuator is effectively reduced by a thermal impulse at the fixing portion between the cylinder body and the cylindrical wall to exert less thermal influences upon the packing of the piston member.