Obtaining of a secure reciprocation of piston magnets and enhance the continuity to obtain a continuous rotary power. The rotary power generating apparatus has the first, second piston magnet members the first, second connecting rods the crankshaft the first, second guide members and the first, second fixed magnet members and it has the demagnetizing member including the demagnetizing rotating board. The first, second piston magnet members and the first, second fixed magnet members are arranged so that the top pole surfaces and fixed pole surfaces having the same polarity, opposes each other. The demagnetizing rotating board has demagnetizing magnet parts, having magnetic forces weaker than the magnetic poles of the top pole surfaces a and different from the polarity of the top pole surfaces and the non-magnetic force parts.