The present invention provides a windowpane defogging device for a motor vehicle which, upon generation of a fog on a windowpane, comes into a defogging mode to remove the fog generated on the windowpane. The windowpane defogging device includes a relative humidity detecting unit configured to detect a windowpane relative humidity of a driver seat view field region of the windowpane and a windowpane relative humidity of a passenger seat view field region of the windowpane, and a control unit configured to control entry into the defogging mode by determining the generation or non-generation of the fog on the windowpane based on the windowpane relative humidity of the driver seat view field region and the windowpane relative humidity of the passenger seat view field region inputted from the relative humidity detecting unit.