PURPOSE: To prevent an adverse influence on a network due to the erroneous setting of a device number by automatically setting the device number at a device side without setting the device number by a user.

CONSTITUTION: In a stackable hub, an output signal Q1 of the first stage of a shift register 21 is inputted to the shift register 21 of the hub at the next stage. Then, a self-number recognizing part 22 recognizes which strobing a position where the output of the first stage of the shift register 21 is generated corresponds to. Thus, the self-number can be automatically decided as a value obtained by successively adding only 1.