is a low idling rotation speed when the angle u03b2 of the composition vector t a is larger than u03c61 and not more than u03c62 the reduction rate is maintained at 100 when the angle u03b2 of the composition vector t a is larger than u03c62 and not more than u2212 u03b1 the reduction rate is reduced following the increase of the angle u03b2 at u2212 u03b1 the reduction rate is 0 that is the engine rotation speed n a is the engine rotation speed calculated at the step s 40 herein u03c61 and u03c62 are angles are linearly symmetrical with u2212 90 u00b0 for example when u03c61 is u2212 100 u00b0 u03c62 is u2212 80 u00b0 when the angle u03b2 of the composition vector t a is larger than u03b1 and not more than u03c63 the reduction rate is increased following the increase of the angle u03b2 at u03c63 the reduction rate is 100 that is the engine rotation speed n a is the low idling rotation speed when the angle u03b2 of the composition vector t a is larger than u03c63 and not more than u03c64 the reduction rate is maintained at 100 when the angle u03b2 of the composition vector t a is larger than u03c64 and not more than 180 u00b0 u2212 u03b1 the reduction rate is reduced following the increase of the angle u03b2 at 180 u00b0 u2212 u03b1 the reduction rate is 0 that is the engine rotation speed n a is the engine rotation speed calculated at the step s 40 herein u03c63 and u03c64 are angles are linearly symmetrical with 90 u00b0 for example when u03c63 is 80 u00b0 u03c64 is 100 u00b0 u03c61 u03c62 u03c63 and u03c64 can be changed.