In the disclosed solution sand to be cleaned is thermally cleaned by rotating the sand being cleaned in a large oven (1) by rotating the oven (1). Before cleaning, the sand may be pre-processed by crushing any lumps and cleaning the sand fraction by magnetic separation. Preprocessed sand to be cleaned and heat energy are fed (5) into the rotating oven. The oven (1) is set slightly inclined so that a second end of the oven (1) is lower than a first end. The inclination and rotating speed of the oven (1) as well as the feed amount of sand are adjusted, whereby the advancing speed of the sand may be adjusted, as well as the ratio of the sand being cleaned to the volume of the oven (1) kept as desired. The temperature of the oven (1) is monitored at the coldest area of the oven, which is substantially at the second end of the oven. The temperature of the oven (1) is adjusted by adjusting the amount of heat energy fed in. By means of temperature monitoring and knowing the advancing speed of the sand, it is also possible to determine the average temperature of the sand and adjust it as desired by adjusting the supplied heat energy. Finally, the cleaned sand is let run (12) from the second end of the oven (1).