Abstract: The engine balancer unit includes a first oil pan attaching to the bottom of an engine cylinder block, a second oil pan attaching to the first oil pan to close a bottom of the first oil pan, and a unitary case inserted in the first oil pan to support counterbalance shafts with balance weights. The cylinder block does not need to be modified to install the engine balancer unit because the first oil pan which encloses the unitary case is attached under the cylinder block. Further, fluid tightness between the first oil pan block is not spoiled by the unitary case because the unitary case is provided inside the first oil pan. Furthermore, fluid tightness between the first oil pan and the second oil pan is not spoiled by conventional fastening structure.