To easily and economically implement reforming of a door into a sliding door, by using an existing door body.

A supporting unit 40 is attached to one of jambs of a door frame, and a sliding guide 50 is attached to a head of the same. The supporting unit 40 consists of a supporting shaft 41 attached to the one jamb along the same, rotary arms 42 and 42 radially outwardly extending from the supporting shaft 41, and an attachment base 43 rotatably supported at tip portions of the respective rotary arms 42 and 42. By attaching the attachment base 43 to a widthwise central portion on a surface of the door body 10, the door body 10 is pivotally and rotatably single-supported, whereby a side edge on the side of the other jamb of the door body 10 is supported by the sliding guide 50 in a linearly movable manner.