An automatic tape threading device for a magnetic recording and reproducing system wherein a leading tape member of a width larger than that of a magnetic tape for threading the magnetic tape in a cartridge through a path of travel of the magnetic tape including the outer circumference of a drum mounting a rotary magnetic head includes a joint member connected to one end portion of the magnetic tape in the cartridge and another joint member connected to one end portion of the leading tape member. The two joint members are brought into engagement with each other conjointly with the attaching of the cartridge to the magnetic recording and reproducing system, and then a takeup reel is actuated to withdraw the magnetic tape from the cartridge through the leading tape member, so as to thereby thread the magnetic tape through the path of travel of the magnetic tape.