A bonnet is pressed and fixed to a valve body using a pressing means of an attachment fixed to the valve body, and also a coupling means is released from coupling, and a work housing is attached to the attachment, the pressing means is released from pressing and fixing, the bonnet is taken out to an inside of a storage space of the work housing and is removed, and thereafter a short pipe that has an inner plug is attached to the valve body, the short pipe is pressed and fixed to the valve body using the pressing means of the attachment, the work housing is removed from the attachment, the short pipe and the valve body are coupled and fixed to each other in a sealed state using a second coupling means, and the attachment is removed from the valve body.