To provide a mold open/closure device of a concrete form wherein a mold can be easily opened in the form provided with a core.

A form 1 comprises side plates 3 and a core 5 provided within the side plates 3. A lifting rod 9 is connected to the core 5 so as to be movable vertically. A core lifting mechanism 44 comprising a cam mechanism which lifts the rod 9 to lift the core 5 is provided for a rotary body 31 and the rod 9. A side plate form open/closure mechanism 45 comprising a cam mechanism for opening the side plates 3 outwardly is provided. Axes 37A are provided in a rotary arm 35 integral with the rotary body 31 and a cam groove 36 is formed in the side walls 3. If the body 31 is rotated so that the core 5 is lifted in a form opening direction by means of the mechanism 44, the side plates 3 open the form in association with the lifting action of the core.