PURPOSE: To arrange core shearing reinforcing bars so that they are not in the way of the placement of inside forms by dismantling side forms after placed concrete exhibits its specific strength and, at the same time, discharging fluid filled in the hollow inside forms to strip them.

CONSTITUTION: A plurality of main beam bars 3 opposed to each other up and down are placed in opposite side forms 2 at a required interval and, at the same time, outside circumferential shearing reinforcing bars 4 consisting of square hoops are placed to the outside circumference of the main beam bars 3 at a required interval. In addition, core shearing reinforcing bars 5 are placed between the main beam bars 3 and 3 opposed to each other in the central part. Then, bag-shaped hollow inside forms 7 capable of expansion and contraction are inserted and filled in spaces 6 partitioned with the outside shearing reinforcing bars 4 and core shearing reinforcing bars 5 in a state of contraction. After that, the insides of them are filled with fluid to expand, and a shape as an inside form is maintained while keeping the hollow inside forms 7 in a state of adherence to each other. In such a state, concrete 8 is placed in the side forms 2, the side forms 2 are dismantled after the concrete 8 exhibits its specific strength to solidify and, at the same time, the fluid in the hollow forms 7 is discharged and the form 7 is released.