An outboard engine system includes a case member which is disposed below an engine body and integrally provided with an oil pan opening upwards, and a passage wall extending vertically to define an exhaust gas passage through which an exhaust gas flows. A portion of an outer wall of the case member is formed by the passage wall, and a recess is provided in the outer wall of the case member with a portion of the passage wall facing the recess. The recess is covered with a plate member fixed to the outer wall of the case member, and a cooling water passage, through which cooling water flows, is defined between the plate member and the recess. Thus, the passage wall integral with the oil pan is cooled effectively, while avoiding increases in size and weight.