An internal combustion engine provided with a cylinder block which can move relative to a crankcase is provided with a block movement mechanism arranged at just one side of the left and right of the internal combustion engine, a support member supporting a side surface of the cylinder block, and a pushing member pushing against a side surface of the cylinder block at the opposite side to the side surface supported by the support member. Further, the support member supports the side surface of the cylinder block at the side of arrangement of the block movement mechanism, while the pushing member pushes against the side surface of the cylinder block at the opposite side to the side of arrangement of the block movement mechanism.