A magnet type rodless cylinder comprised of cylinder tube of a double tube structure provided with an outside tube of an elliptical flattened outside circumferential shape and two inside tubes inserted into the outside tube. When pressurized air is supplied alternately to the inside tubes from a port provided at an end cap, two pistons move reciprocatingly inside the inside tubes. Due to this reciprocating motion, this reciprocating motion causes reciprocating motion of a slide magnetically coupled with the two pistons at the outside of the outside tube. The inside pressure caused by the pressurized air acts exclusively on the inside tubes and does not directly act on the outside tube of the flattened outside circumferential shape, so the thickness of the outside tube can be reduced and, even if the cylinder tube is made a double tube structure, the total thickness will not become greater than in the past. For this reason, it is possible to provide a practical flattened type of magnetic cylinder with a small height without greatly increasing the magnetic coercive force.