To automatically operate by high power accompanying increase of external force, during operating by low power with the use of low pressure fluid (oil, air, or the like).

A primary side cylinder chamber in which a piston 4 integrated with a rod 11 closing a passage 9 is fitted, is provided with a primary side fluid chamber 61 and a primary side liquid chamber 62, a pipe line 5 is communicated to the primary side fluid chamber 61 and to the primary side liquid chamber through a working valve mechanism and further actuates the working valve mechanism, next, a secondary side cylinder chamber in which a piston 10 integrated with a rod projecting outward is fitted, is provided with a secondary side liquid chamber 63 and a secondary side fluid chamber 64 communicated to a pipe line 6, and the secondary side liquid chamber 63 is communicated to the primary side liquid chamber 63 through the passage 9. Further this cylinder device is provided with a supply fluid chamber fitted with a piston and communicated to the respective pipe lines, a working liquid tank having a working liquid chamber communicated to the working valve mechanism, and a returning liquid tank having a returning liquid chamber communicated to a valve mechanism for returning.