It is a technological object of the present invention to provide an information processing device, a card and a card system that have a high level of security. In order to achieve the object described above, the present invention provides a data processing apparatus comprising at least a first information processing device and a second information processing device connected to the first information processing device by a signal line, the data processing apparatus having a means for changing power consumption on the signal line during transmission of a signal through the signal line in accordance with an actual state of the power consumption that would be observed when the means were not used. Furthermore, the present invention also provides another data processing apparatus comprising at least a first information processing device and a second information processing device connected to the first information processing device by a signal line wherein, between at least either the first information processing device or the second information processing device and the signal line, a signal from the first information processing device or the second information processing device can be encrypted and a signal received from the signal line can be decrypted. In addition, the present invention also provides a further data processing apparatus comprising at least an information processing device, an information memory device and a signal line connected at least to the information processing device wherein at least in an operation to store information into the information memory device, the information is encrypted, and information stored in the information memory device can be decrypted.