Abstract: To render a formwork (10), having a hollow (11b) defined therein and embedded in the elongated concrete member (40) during casting of a concrete material, to be lightweight, environmentally friendly and inexpensive and to have a sufficient strength, the formwork (10) includes a tubular member (11) formed by bending an oblong plate of corrugated cardboard (5) made of paper. The corrugated cardboard plate (5) includes a plurality of corrugated cardboards (3) laminated together and each formed by bonding liner sheets (2) to surfaces of a corrugated sheet (1). The corrugated cardboards (3) are laminated together with flutes of the corrugated cardboards (3) oriented in a direction (P) across a thickness of the corrugated cardboard plate (5), and a sheet (4) is bonded to front and rear major surfaces thereof.