Abstract: A stepless speed change device for cutting metal-working machines includes a drive gear set, a driven gear set, an intermediate gear set and two transmission belts. Each gear set includes an upper disk and a lower disk which are inter-engageable, and a resilient seat disposed therebetween so that the upper disk and lower disk may inter-engage and the upper disk may resiliently displace to open or close an opening defined between the upper and the lower disks. The intermediate gear set includes a first adjusting gear and a second adjusting gear respectively at an upper end and a lower end thereof such that when the first adjusting gear is in an open state, the second adjusting gear is in a closed state, and two rotating devices for controlling the first and second adjusting gears to synchronously displace therewith to control the size of openings through which the transmission belts pass.