PURPOSE: To form a groove in the direction crossing to the direction for pulling a concrete block by retractably providing a joint strip which has both the part projected from a guide groove and the part held into the guide groove to the guide groove opened to the inside of the formwork main body.

CONSTITUTION: A partition wall 14 for molding two concrete blocks is fixed to the center of the formwork main body 12 of a formwork 10. A guide groove 8 of the direction crossing to a projected line 16 is formed to the midway part of the partition wall 14. A joint strip 24 which is moved back and forth through the insides of the guide groove 18 and a guide pipe 22 is constituted of a guide part 24a and a joint part 24b and connected with a cylinder 26 and freely retractably controlled. Furthermore the bottom face of the formwork main body 12 is opened and closed by a raiseable closing plate 28. Thereby a lateral groove of the direction crossing to the direction for pulling the concrete block can be easily formed.