A device for cleaning a collector of a solar thermal electric power generation system is provided which does not need to secure any water source or the like and which improves the flexibility of selection of a place for installing the solar thermal electric power generation system. The cleaning device includes an air supply device and an air nozzle configured to blow air supplied from the air supply device, which are mounted on a self-propelled truck. The cleaning device is capable of blowing air blown from the air nozzle against a sunlight reflecting surface of a collector to clean up the sunlight reflecting surface. The air nozzle is configured to be capable of moving up and down. The cleaning device further includes a soiling degree measuring device for measuring soiling degrees of the sunlight reflecting surface before and after air blowing by the air nozzle.