PURPOSE: To permit the reciprocating motion utilizing solar heat by a method wherein heating medium is sealed into a bellows and the radiation heat is projected alternately against the heating medium under utilizing a shielding tube.

CONSTITUTION: When the projection of the solar heat against the bellows 7 is continued, the temperature of the heating medium sealed into the bellows 7 is risen gradually and the volume of the same is expanded, therefore, a pressure in an enclosed chamber 8 is increased, disc springs 11, 12 are converted and a rod 3 is slided. Further, the left side bellows 7, which has been in a contracted condition until this time, is expanded while the right side bellows 9, which has been in an expanded condition until this time, is contracted. At the same time, the shielding tube 13 moves to the left to cover the left side bellows 7 and, thus, the reciprocating motion is effected.