A stepless automatic speed changer whose structure is simplified and manufacturing is reduced transmits the power from an input shaft to an output shaft without clutching or changing gears. The speed changer comprises the input shaft (12); an input sun gear (16) formed between a first part (12A), a second part (12B) and a third part (12C) of the input shaft (12); a speed change shaft (18) equipped in the second part (12B) of the shaft (12); an output shaft (26) equipped in the third part (12C) of the shaft (12); a carrier (30) rotating independently of the shaft (18); a carrier (34) rotating independently of the shaft (26); a multiple of planetary gears (42) rotatably equipped in a fixed pin (36); a forward brake system (59) pushing a braking force on the carriers (30,34); a backward brake system (99) pushing a braking force on the speed change shaft (18).
