PURPOSE: To enable the whole process of assembling a long size precast formwork and of demolding to be performed by one processing, by providing actuators for tilting side forms outward, and mounting one of the side forms on a truck device for retracting the side form to a lateral side of a bottom plate.

CONSTITUTION: A forward 1 comprises a bottom plate 2 and side form 3a, 3b, and bottom surfaces of the side forms are made to make contact with the top surface of the bottom plate 2. The side forms 3a, 3b are provided with actuators, 5a, 5b for tilting the side forms outward, respectively. The tilting angle of the side forms is preferably 15W45°C. After the side form 3b on one side is tilted by the actuator 5b, a truck device 9 is advanced and retracted, thereby enabling the operator to enter and operate between the bottom plate 2 and the side form 3b. By remote control of compressed air supplied to each cylinder for the formwork through using solenoid valve, it is possible to remote ly effect assembly of the side forms and demolding at a stroke, so that labor saving and noise reduction can be contrived.