A stepless speed change device comprises an eccentric cam assembly whose eccentricity is adjustable relative to a center shaft, a carrier rotatably arranged about the eccentric cam assembly, rotary members on input and output sides, respectively, and a plurality of friction pieces arranged in two rows each including a plurality of the friction pieces. The friction pieces in one row are rockably connected to the rotary member on the input side and free ends of the friction pieces are engaged with an inside of the carrier. The friction pieces in the other row are rockably connected to the carrier and free ends of the friction pieces are engaged with an inside of the rotary member on the output side. As an alternative, all the friction pieces are rockably connected to the carrier and free ends of the friction pieces in one row and the other row are engaged with insides of the rotary members on the input and output sides, respectively.