To provide a stabilizer to reduce the deflection of a laterally-collapsible wheelchair in traveling on an uneven or bumpy surface.

Two spindle tubes 1 and 2 are pivotally connected to form an X shape. Parallel stabilizing tubes 4 and 5 are pivotally attached to parallel stabilizing tube anchors through parallel stabilizing tube mounting screws 7. Extensible tubes 9 are extensibly inserted into both ends of the spindle tubes 1 and 2. Elbow tubes 12 and 14 are rotatably fitted into each connecting tube of the extensible tubes 9. Each elbow tube is provided with a stopper 17 for support tube mounts on one lateral side, as well as with a lateral frame support tube mount 15 via a support tube mount fixing screw on the other lateral side. The lateral frame support tube mount has a support tube mount fixing screw at the other end. The shafts and tubes are attached to the vertical portion of the lateral frame in a collapsible wheelchair from the bottom via the lateral frame support tube mounts so as to stabilize the collapsible wheelchair.