1. A position adjustment method of a vehicle display device including: a display device configured to display a virtual image on a front field of view of a driver, and a viewpoint detecting camera configured to capture an image of the driver, the method comprising: installing a measuring camera in a position at which a calibration pattern for acquiring information for the virtual image and position adjustment can be captured, and disposing the calibration pattern in a predetermined position with respect to a viewing angle of the virtual image, an angle of view of the viewpoint detecting camera, and an angle of view of the measuring camera; capturing the calibration pattern by the viewpoint detecting camera and the measuring camera; capturing the virtual image by the measuring camera; calculating a relative position and a relative orientation between the measuring camera and the viewpoint detecting camera, and calculating a relative position and a relative orientation between the measuring camera and the display device by using an image captured by the viewpoint detecting camera and an image captured by the measuring camera, and calculating a relative position and a relative orientation between the viewpoint detecting camera and the display device from the two relative positions and orientations as orientation information; and adjusting a position of the virtual image displayed by the display device by using the orientation information.