PURPOSE: To correct the displacement of a focus due to the revolution of the earth by forming one part of a condenser lens by a liquid lens and injecting a liquid into the liquid lens and discharging it from the lens.

CONSTITUTION: The condenser lens 6 condensing beams to a heat collecting tube 3 is formed by a base lens 7 and the liquid lens 8 of the lower section of the lens 7, and the focal distance of the condenser lens 6 is adjusted through the injection of the liquid into a liquid injection port 9 and its discharge from the port 9. The heat collecting tube 3 and the condenser lens 6 are inclined at angles equal to the latitude of positions to be mounted from south to north and set up. The liquid is injected and beams are condensed to the heat collecting tube 3 as shown in Fig. 6 (a) in the vernal equinox or the autumnal equinox, and the liquid is discharged and the focal distance is lengthened as shown in Fig. 6 (b) when the altitude of the sun changes and sunbeams are projected to a beam receiving surface at angles on the summer solstice or the winter solstice, thereby condensing beams onto the heat collecting tube 3.