is closed up a user rotates the knob 300 and then the coupling projection 301 arranged on the knob 301 slides in the sliding grooves 401 and 402 of the link 400 by the rotating knob 300 and rotates the link 400 accordingly the rotating link 400 makes the door damper 200 which has a same rotary axis and whose end is coupled with the link rotated around the axis if the duct inside as shown fig2 or 3 is closed up by the rotation of the damper door 200 the pressure of fluid is applied to the damper door 200 at this time the power applied to the damper door 200 is transferred to the knob 300 by the link 400 at this moment the coupling projection 301 of the knob 300 is located in the end of the sliding grooves 401 and 402 of the link 400 and then the round coupling projection 301 is surface contacted with the end of the half rounded sliding grooves 401 and 402 because of the surface contact numberless many points exist but the points located in the farthest from the rotary center of the knob among such contact points builds up one line and the normal line of those indicates the direction of the power which is applied to the knob.