An upper oil pan of an engine includes a flange with an oil pan abutment surface that is fixedly coupled to a flange formed on a cylinder block. The oil pan abutment surface comprises a flat portion that is substantially parallel to a cylinder block abutment surface of the cylinder block, a slanted portion that is slanted with respect to the flat portion, and a vertical wall portion connecting the flat portion and the slanted portion. The vertical wall portion is formed substantially perpendicular to the flat portion. The cylinder block and the upper oil pan are fixedly coupled together via an interposed liquid gasket. Thus, the seal structure in which the liquid gasket is adequately cured is provided, and sufficient sealing properties can be obtained with a small amount of material used.