PURPOSE: To inspect and maintain a front and back changeover valve without disassembling a back lifting cylinder by providing the third hydraulic oil passage connecting to the frontal lifting cylinder and the front and back changeover valve for selectively communicating the first hydraulic oil passage with the second and third hydraulic oil passages in a front and back changeover block.

CONSTITUTION: A distribution block 29 is joined to the right side of a back lifting cylinder 17 with bolts and a front and back changeover block 30 is joined to the back surface (R) of the distribution block 29 with bolts. The front and back changeover block 30 is provided with the first and second poppet valves 641 and 642, a cam shaft 65, a slow lowering valve 66 and an overload escape valve 67 and the first hydraulic oil passage 44 is selectively communicated with the third hydraulic oil passage 47 connecting to a front lifting cylinder 14 and the second hydraulic oil passage 45 through the poppet valves 641 and 642.