To provide a simple hydraulic rodless cylinder having excellent environmental resistance, wherein an external chain can be omitted.

This hydraulic rodless cylinder is provided with a simple shape pressure-resistant cylinder parallel to a cylinder 10 fixed to housings 30 of both the ends instead of conventional storage chambers on free end sides of an internal chain 26 perfectly separated into two such that an endless shape can be configured by only the internal chain 26. A small diameter rod is provided inside the pressure-resistant cylinder on the opposite side of a piston and is connected to the internal chain, and a pressure-resistant cylinder center part is provided with a partition slidably sealing the rod.