PURPOSE: To easily perform a timbering work for a concrete structure on the water by a method in which a float is detachably attached to a timbering frame, the frame is floated on the water surface and fixed to a pile, the float is removed, and then concrete is placed into a formwork assembled on the frame.

CONSTITUTION: When constructing a structure on the water, e.g., landing pier, etc., plural steel tubular piles 31 are erected on the sea bed, and a timbering frame 20 of an almost oblong hollow and square cylindrical steel assembly, having a float 21 on its lower part and a stepladder 25 on its upside, is towed to between the piles 31. Then, when the water level is higher than the lowest tide level L2, the frame 20 is connected hanging steel bars 32 between the upper ends of the piles 31. When, under the condition, the tide level is lowered and an aperture is formed between the float 21 and the flat face, a wire 23 is cut off to cause the float 21 to fall for recovery. After this, a formwork 41 is assembled through a proper square timber 40 on the stepladder 25, and then concrete 42 is placed and supported on the piles 31 to contruct a concrete structure on the water.