A mounting system that is capable of effectively draining the rainwater off the roof, while enabling easy installation of the solar battery modules. The mounting system is adapted to install on the roof an array of the solar battery modules each of a panel-like configuration with a rectangular frame. The system includes a plurality of vertical rails which are secured to the roof and extend along a roof sloping direction towards eaves from a ridge of the roof for holding the solar battery modules between a laterally spaced pair of the vertical rails in a spaced relation from a roof sheathing. Each of the vertical rails is formed on opposite sides thereof respectively with stepped shoulders which extend the full length of the vertical rail for supporting thereon the frame of the laterally adjoining solar battery modules. The vertical rail is further formed on opposite sides thereof with respective trough sections which extend the full length of the vertical rail and are disposed below and outwardly of the corresponding stepped shoulders for receiving the rainwater sneaking along the frame. Thus, the rainwater is effectively prohibited from reaching the roof sheathing and advancing through the roof into interior of the house.