According to the rodless power cylinder of the present invention, a pressure relieving passage which opens to the space between the inner seal band and the outer seal band at one end, and to the outside of the cylinder barrel at the other end is provided in the end cap of the cylinder barrel. Further, a cap cover is provided on the end cap. The cap cover includes a valve element portion which resiliently fits into the end of the relieving passage opening to the outside. When the pressure in the space between the seal bands increases to a predetermined level due to leak of the inner seal band, the valve element opens outward due to the pressure in the relieving passage to allow the fluid in the space to escape to the outside. Thus, the pressure in the space between the seal bands are kept at a low level even if a leak in the inner seal band occurs and, thereby, a blow off of the outer seal band and damage to the rodless power cylinder is prevented.