A head unit position adjustment method for adjusting a position of a second head unit attached adjacent in a width direction to a first head unit fixed to a base member, wherein the head unit position adjustment method comprises: (a) temporarily attaching the second head unit in an attachment region adjacent to the first head unit in the width direction; (b) causing a tip part of a reference pin to stand in a surface normal direction of the base member with respect to the base member by mounting a base end part of the reference pin into a reference pin hole provided adjacent to the attachment region in the base member; (c) mounting an adjustment jig on the base member, the adjustment jig being configured to support the second head unit movably in the width direction and rotatably about the tip part of the reference pin as a center of rotation while engaging the tip part of the reference pin; (d) inserting a width direction adjuster between the first head unit and the second head unit and inserting a rotation direction adjuster between the second head unit and the adjustment jig at a separation position distant from the reference pin in the width direction; (e) adjusting a gap between the first head unit and the second head unit in the width direction by moving the second head unit in the width direction by the width direction adjuster with the temporary attachment of the second head unit loosened and adjusting a rotation direction position of the second head unit with respect to the tip part of the reference pin by moving the second head unit in an orthogonal direction orthogonal to both the width direction and the surface normal direction at the separation position by the rotation direction adjuster; and (f) fastening and completely attaching the second head unit to the base member after the completion of the operation (e). (a) temporarily attaching the second head unit in an attachment region adjacent to the first head unit in the width direction; (b) causing a tip part of a reference pin to stand in a surface normal direction of the base member with respect to the base member by mounting a base end part of the reference pin into a reference pin hole provided adjacent to the attachment region in the base member; (c) mounting an adjustment jig on the base member, the adjustment jig being configured to support the second head unit movably in the width direction and rotatably about the tip part of the reference pin as a center of rotation while engaging the tip part of the reference pin; (d) inserting a width direction adjuster between the first head unit and the second head unit and inserting a rotation direction adjuster between the second head unit and the adjustment jig at a separation position distant from the reference pin in the width direction; (e) adjusting a gap between the first head unit and the second head unit in the width direction by moving the second head unit in the width direction by the width direction adjuster with the temporary attachment of the second head unit loosened and adjusting a rotation direction position of the second head unit with respect to the tip part of the reference pin by moving the second head unit in an orthogonal direction orthogonal to both the width direction and the surface normal direction at the separation position by the rotation direction adjuster; and (f) fastening and completely attaching the second head unit to the base member after the completion of the operation (e).