To provide a drive mechanism part of a motor-driven curtain opening/ closing device which is miniaturized for easy installation, while widening the effective opening width of a curtain and ensuring the transmission of a driving force to prevent the slip of a curtain pull cord.

In the drive mechanism part of a motor-driven curtain opening/ closing device, an electric motor 4 is provided within a drive mechanism part case 3 and in a position biased to one side of the extension centerline (y) of a rail 1, and a motor-side pulley 6 is provided so that a shaft 8 to which the pulley 6 is pivoted is located in a position opposite to the electric motor 4 across the extension centerline (y) of the rail 1. A pulley 9 for guiding a pull cord is provided near the side end of the rail 1 and on the extension centerline (y) of the rail 1, and a pull cord 7 hung in a parallel position between the pulley 9 and a driven pulley is wound on the motor side pulley 6 while crossing each side of the guide pulley 9.