To detect leak of hydraulic oil from a lower chamber having the hydraulic oil stored therein to an upper chamber without storing the hydraulic oil.

A vertically-arranged cylinder 2 where an upper chamber 24 is separated from a lower chamber 25 by a piston 23, hydraulic oil is stored in the lower chamber 25, and no hydraulic oil is stored in the upper chamber 24 includes: a float plate 3 movably arranged along a piston rod 27 in the upper chamber 24, and structured to float on the hydraulic oil when the hydraulic oil has leaked to the upper chamber 24; a distance sensor 4 arranged on an upper cover 21 for detecting a separation distance to the float plate 3; an end detection sensor 5 for detecting a shortened-side stroke end of the piston rod 27; and a leak determination means 6 to determine leak of the hydraulic oil to the upper chamber 24. When the end detection sensor 5 detects the shortened-side stroke end, the leak detection means 6 determines the hydraulic oil to leak to the upper chamber 24 when the separation distance detected by the distance sensor 4 is smaller than a specified value (L1).