The invention relates to a flap device which includes a flap case enclosing a channel for fluid to pass through, a flap pivoting around a oscillating axis to control a cross section of the channel for fluid to pass through, a adjustment device for driving the flap and having a drive shaft, a coupling device for coupling a flap shaft and the drive shaft, wherein the flap shaft is oppositely rotary to a flap shaft resistance and coaxially extends with the oscillating axis, and the drive shaft is coaxially oriented to oscillating axis, so that the linear motion of the drive shaft transforms to a rotary motion of the flap shaft. The coupling device is provided with a guide case which has a spiral guid part; the spiral guide part has spiral guide groove with two mutually faced guide planes and a sliding body inserting into the groove; the guide shell is relatively rotarily connected with the flap shaft resistance, accordingly, the sliding body is oppositely rotarily connected with the drive shaft resistance; when corresponding sliding body and the guiding groove are complementarily and spirally oriented and have two back-to-back sliding surface for adhering to one of the guiding surface, the wear is reduced.