Guide device and scarfed surface forming method

Provided is a router guide that comprises: a guide part that has a first through-hole and a guide surface that guides a cutting tool attachment part of a router device along the first through-hole; a maintaining part that maintains the position of the guide surface; and a positioning part that positions the guide part. The guide surface is formed such that， in a state where the maintaining part is in contact with a surface of a composite material， the distance from the surface becomes smaller as the guide surface approaches a second through-hole. The guide part is formed so as to be elastically deformable. In a case where the guide part is caused to elastically deform along the surface shape of the composite material， the maintaining part maintains the position of the guide surface with respect to the surface at a position that corresponds to the surface shape.