PURPOSE: To improve strength of a cylindrical block by forming a heaped-up portion of extra concrete before the concrete becomes hard, and by removing the heaped-up concrete portion outside together with a formwork.

CONSTITUTION: A formwork 14 is assembled by erecting a side plate 1 and an end plate 2 on a base stand 13 to form an outer mold and jointing and clamping the bottom frame 8 of a medium mold 5 thereon. A concrete 16 is then poured into a hollow portion 15 in the formwork 14 while being vibrated and compacted. The bottom frame 8 is removed from the formwork 14 after elapse of 10 to 20 minutes from completion of concrete pour in. A heaped-up portion 19 is formed by pouring an excessive amount of mortor 18 into the exposed portion. The bottom frame 8 is pushed in the horizontal direction to push out the heaped-up portion 19 outside of the formwork 14. Lastly the bottom frame 8 is secured to the medium mold 5 and the concrete 16 in the formwork 14 is hardened. Air bubbles and foams generated on the bottom surface portion of a medium mold 8 are then removed.