To provide a crankshaft supporting structure of an engine in which the total width dimension of particularly the crankcase portion is reduced and the increase of vibration and noise by an unbalance force between crankshafts are prevented by increasing the supporting rigidity of the crankshafts.

One and the other side crankshafts 11, 12 are disposed at the positions apart from the center crankshaft 10 to the anti-cylinder side. A crankshaft supporting member 19 comprises a first supporting member 17 and a second supporting member 18 disposed between the first supporting member 17 and a case side supporting part 2e. The one and the other side crankshafts 11, 12 are supported on a first split surface of the first supporting member 17 and the second supporting member 18. The center crankshaft 10 is supported on the second split surface of the second supporting member 18 and the case side supporting part 2e. The first and second split surfaces are formed to extend parallel to each other. The second supporting member 18 is formed on the center side and one and the other side crankshafts 11, 12 by one common member.