The laminar longitudinal vortex rolls driven by the gravity currents between horizontal parallel plates where temperature were not uniform in the lateral direction normal to the forced flow have been investigated by means of a flowvisualization technique and 3-D numerical calculation. The difference in magnitude of gravity currents along the horizontal plates was found to cause three different kinds of vortex rolls. The mechanism of the vortex-roll formation and its effects on the local Nusselt number were made clear from the numerical prediction.