To provide a hydraulic system and a forklift having the hydraulic system, capable of early finding a leak of hydraulic oil from a bottom chamber to a rod chamber.

A hydraulic system 1A comprises a hydraulic cylinder 2 of an internal drain type and a hydraulic device 10A supplying hydraulic oil to a bottom chamber 2d of the hydraulic cylinder 2. The hydraulic device 10A includes: a sliding amount detection means 15 for detecting a sliding amount of a piston 2b; a pressure detection means 16 for detecting a pressure of a rod chamber 2e at a predetermined period; a leak determination means 17 for determining the presence of leak of the hydraulic oil from the bottom chamber 2d to the rod chamber 2e on the basis of the detection results of the sliding amount detection means 15 and the pressure detection means 16; and an alarm means 18 for issuing an alarm when the presence of leak is determined by the leak determination means 17.