PURPOSE: To simplify the operations as well as to save power by a method in which a telescopic concrete-placing tube is contracted as the placing operation of underwater concrete proceeds.

CONSTITUTION: A crane 1 is set on a caisson 11 settled on the seabed at deep water depth and a concrete-placing tube 3 telescopically connected with many tubes 3a of different diameters is supported on an injector 2 hung down from the crane 1. The tube 3 is wholly extended and underwater concrete is placed. The tube 3 is then wholly contracted, during which period underwater concrete is continuously placed. Underwater concrete can thus be placed without discontinuation with high operating efficiency.