PURPOSE: To make a belt maintain a constant tension, even when the belt has elongated, in order for the compressor to rotate at a proper number of revolutions by a mechanism wherein one end of a tension spring is pivoted at a midpoint on a spring-holding arm which is pivoted at an end of a tension arm and at the other end of the tension spring a second bridging arm is pivoted.

CONSTITUTION: A compressor C is supported at a part on a pivot 46, the other end of which is given force in the direction of the stretch of a V-belt 37 by a tension spring 44. When the V-belt 37 elongates, the compressor C turns with the pivot 46 as an axis and descends. A tension arm 41 detects this turning, downward move of the compressor C and turns, with the other end of the tension arm 41, a spring-holding arm 40 on which a tension spring 44 is pivoted. The link ratio at which the tension spring exerts force on a pivotal pin 43 is altered from b/a to b'/a', so that the force exerted by the tension spring 44 is made constant irrespective of the elongation of the V-belt 37.