To provide a highly accurate mortar placing method and a device for reducing labor and increasing a speed in that manpower is so far required much when placing mortar on the basin bottom such as a sedimentation base and there is a tendency of lengthening a construction period since a mortar surface becomes nonuniform.

Rails 5, 5 as an installation reference are arranged on a tank wall 80, and a top place of basin bottom rails 3, 3 and a placed mortar surface are uniformed by traveling by suspending a basin bottom rail adjusting machine and mortar adjusting machine with these reference rails as the basis.