To prevent a concrete from falling at opening a form when the concrete having an unstable shape with a biased center of gravity is to be molded.

A concrete form 1 is constituted so as to journal a first and a second side plates 3 and 4 to a bottom form 2 in order to mold the inclined surface 5a of a concrete block 5 inclined taperedly backwards. An abutting plate 50, which locates at the second side plate 4 and abuts against the upper edge part of the concrete block 5 when the second side plate 4 is opened, is provided so as to support the concrete block 5 with the abutting plate 50, resulting in preventing the concrete block 5 from falling down violently.