In a data transmission system having a plurality of apparatuses connected to one another in a cascade, each slave apparatus has an identification number retention section retaining the same identification number as the other slave apparatuses, a receiver receiving data accompanied by an apparatus number, a judgment section judging that the data is destined for itself when the apparatus number coincides with the identification number, while judging that the data is destined for another slave apparatus when the apparatus number does not coincide with the identification number, and a transmitter changing the apparatus number by subtracting or adding when the data is judged to be destined for another slave apparatus, and transmitting the data to another lower-ranked slave apparatus. Thereby, data transmission can be accomplished without automatically setting an apparatus number to each of the plural apparatuses.