To provide a small-power communication device, by which a communication apparatus is miniaturized and manufacturing cost is reduced.

In a master apparatus 10, the field strength of a slave apparatus received by an antenna section 11 is measured in a measuring circuit 13, a slave apparatus 20 having a maximum field strength is detected in a control section 14, and a password data transmitted from the slave apparatus 20 is received. The received password data is compared with a reference password data stored in a reference storage section 15, and the password data is registered in a registration section 16 when they match. Thus, the slave apparatus 20 registered in the master apparatus 10 can be radio-communicated with the master apparatus 10. Since a keyboard for inputting a password, which has been used in registration is dispensed with, the communication apparatus such as the master apparatus 10, the slave apparatus 20 or the like can be miniaturized, and the manufacturing cost of the communication apparatus can be reduced. Since field strength is measured and the slave apparatus having maximum field strength is registered, other slave apparatuses will not be registered by mistake.