at the front end of the front most basic unit a 1 of the framework mechanism 10 or on the upper and lower sides or any one side thereof to be capable of retracting in a vertical up and down direction or a horizontal left and right direction in the present embodiment the driving unit 30 uses as an example a pair of hydraulic cylinders and the cylinders are vertically fixed at their upper and lower ends by pins p 4 to respective pivot members 15 at the upper end of the link 11 and at the lower end of the link 12 it is also possible that the pumps may be located on the driving unit fixing members additionally mounted at the upper end of the link 11 and at the lower end of the link 12 or on the pivot member 15 not shown it is preferred however to use only one pair of synchronously acting driving units 30 in order to enable the entire set of the steel form system to operate in a stable and balanced way when the steel form system is rather large there may be used one pair of the driving units at both the front and the rear of the framework mechanism 10 under certain circumstances when the steel form is small and light however one driving unit 30 is sufficient in the following the operating condition is described as an example of where the construction is more difficult and thus requires use of a functionally more complex tunneling steel form first under the condition as shown in fig1 hydraulic pressure is forced with the hydraulic pump not shown into the cylinder 30 acting.