A connection structure for pressure vessels is configured such that respective mouth pieces are connected to at least one ends of the pressure vessels, and when adjacent mouth pieces are connected to each other in an intersection direction intersecting with the axial direction, the pressure vessels placed in parallel can be connected to each other. The mouth piece includes a connecting portion along the intersection direction intersecting with the axial direction. The connecting portion includes an external screw thread portion formed on a first side thereof, and a nut rotatably locked to a second side thereof and having an internal screw thread portion threadedly engageable with the external screw thread portion.