PURPOSE: To reduce the size of a whole and to improve durability, by a method wherein a pair of cylinders are disposed facing each other on the identical axis, and pistons in cylinders are coupled to a swing shaft, coupled to a crank shaft, through a specified link mechanism.

CONSTITUTION: A pair of cylinders C1 and C2 are placed on the identical axis facing each other, equal side links L2 and L4 are coupled in a rhomboid shape, in a manner to nip it from above and below by means of links L1 and L3, by means of piston pins p1 and p2 of pistons P1 and P2 engaged with the cylinders C1 and C2 and joint pins j1 and j2, respectively. Two cross arms A2 and A2 cross a cross arm A1 in an X-shape at an intersection point between the diagonal lines of the links in a manner to nip the cross arm A1 from above and below by means of the two cross arms A2 and A2, and end parts thereof are pivotally attached to the neutral points of the links L1WL4 by means of arm pins 7 and 8. A swing shaft 1 is secured at an intersection point between the cross arms to the cross arm A1 perpendicularly to its working surface, and the swing shaft 1 is coupled to a Z crank shaft.