that the parabolas forming the cam profiles are symmetrical it should be noted here that the first condition can be established by incorporating the profile obtained by cutting along a circle the surface of a parabolic cylindroid 35 as described above the second condition can be satisfied if the cam surfaces 30a 30b can be expressed by a curve characteristic of a sine wave in the case of this embodiment provided that the rotation angle of the wave cam 30 is u03b8 and the stroke of the piston 16 is h the relationship between the displacement of the centers q1 q2 of the shoes 33 34 and the rotation angle u03b8 can be expressed by the following equation 4 since the cam surfaces 30a 30b of the wave cam 30 are of the same profile in this embodiment only one cam surface 30a will be discussed the rotation angle u03b8 of the wave cam 30 when the piston 16 is at the top dead center is defined as 0 u00b0 the axis z corresponds to the axis l0 of the drive shaft 13 the axis y is parallel to the axis 35a of the parabolic cylindroid 35 constituting the cam surface 30a and the axis x is parallel to the axis 35a.