To lay a new conduit on a service conduit through the pipe-jacking method without cutting off water, for example, in case an old conduit is retrofitted.

The method comprises a water-tight fixing step of a plug device 1 including a hose 17 which extends to the downstream of a renewed section of the established conduit 2 on interior surface on upstream side of the renewed portion of the established conduit 2, a step in which a novel mono tube 10 with a diameter capable of being accommodated into the established conduit 2 is carried in from a manhole 21 provided downstream of the renewed section of the established conduit 2, at that time by temporally cutting off flowing water in the hose 17 and inserting through the hose 17 into the novel mono tube 10, a step of nextly reopening the flowing water in the hose 17 and propelling it to upstream side of the renewed section while flowing water and subsequently continuing to connect, and a step of connecting with the established conduit 2 required not to renew it in water-tight state and taking out in turn the hose 17 and the plug device 1.