first connecting rod and the large end 28 a of the third connecting rod fig1 a and 10b show a crank angle of 270 u00b0 i e a state wherein the first piston 23 is positioned in the exhaust stroke and the second piston 53 is positioned in the compression stroke when viewed from the direction extending from the crank axes 101 102 as in fig1 a the first and third connecting rods 26 28 and the second connecting rod 27 form a v shape the fourth and sixth connecting rods 54 56 and the fifth connecting rod 55 form a x shape and the first crankshaft 16 and the second crankshaft 17 overlap as shown in fig1 b the large end 54 a on the side of the first crankshaft 16 penetrates to the inner side of the first arm 131 the second arm 132 and the third arm 133 of the second crankshaft 17 the large end 56 a on the side of the first crankshaft 16 penetrates to the inner side of fourth arm 134 the fifth arm 135 and the sixth arm 136 of the second crankshaft 17 the large end 55 a on the side of the second crankshaft 17 penetrates to inner side of the second arm 132 the third arm 133 the fourth arm 134 and the fifth arm 135 of the first crankshaft 16.