To easily perform demolding or the setting of a mold without requiring large force by providing the first link provided to one split form, the second link connected to the first link at one end thereof and pivotally mounted on other split form at the other end thereof and guide members arranged at opposed positions.

A molding inner frame 1 forms a concrete block having a square cross section by injecting ready-mixed concrete in the space S between the inner frame 1 and the molding outer frame arranged to the outer periphery of a spacer 3. In performing demolding after an aging period is elapsed, at first, the molding outer frame is detached and, subsequently, when a first link 4 is revolved, the horizontal U-shaped member 9a constituting a second link 9 is moved and a ring-shaped part 14 is revolved to draw the second link 9 toward the first link 4. At this time, both split forms 1a, 1b are guided by a guide member 17 to mutually approach smoothly. As a result, both split forms 1a, 1b are separated from the inner edge of the spacer 3 to demold a concrete block.