An engine having a crank case of a variable volume may include a crankshaft that may be disposed in a crank case and may be connected to a piston through a connecting rod, which reciprocate in a cylinder, a balance shaft that may be disposed in the crank case and may be operated by the crankshaft, a chamber that may be extended from the crank case, and a balance weight that may be formed on the balance shaft and selectively opens or closes a fluid communication between the chamber and the crank case according to a position of the piston.