To arrange a balancer shaft in relation to a crankshaft so as to meet the requirement in an engine to be used.

In an engine 4, a cylinder 43 is provided in an inclined condition in the stern direction, an exhaust pipe 44 is led upward of a crankshaft 40 from the cylinder 43, the crankshaft 40 is horizontally arranged on a hull so as to be directed in the ship breadth direction, and respectively supported by a pair of bearings in the vicinity of a center part part and in the vicinity of each end part, on air intake tube 6 and an air intake box 61 are arranged on the inclination side opposite to the cylinder 43, a balancer shaft 5 is supported by a bearing at each end part and arranged parallel to the crankshaft 40 below the air intake tube and box, a gear is provided on the crankshaft 40, and the balancer shaft 5 is interlockingly connected to the craned shaft 40 at the same speed in the direction opposite to each other by engaging the gear with gear provided on the balancer shaft 5.