The thermal tides in the lower atmosphere of Venus are examined. It is shown that both the diurnal and semidiurnal tides excited in the cloud layer propagate to the ground, and the existence of thermal tides below the cloud bottom can be attributed almost to the solar heating in the cloud layer. At altitudes of 0--10 km, the atmospheric superrotation is accelerated and decelerated by the momentum transport associated with the diurnal and semidiurnal tides, respectively. The effect of diurnal tide is much smaller than that of the semidiurnal tide, so that the mean zonal flow is accelerated in the direction opposite to the Venus rotation there. It is argued that this momentum transport by the semidiurnal tide is balanced with the surface friction in the atmospheric layer adjacent to the ground, and the net momentum is supplied from the solid Venus to maintain the atmospheric superrotation.