PROBLEM TO BE SOLVED: To solve the problem that it is difficult to attain both the prevention of leaving a mobile phone behind and convenience that function restriction is applied in use. SOLUTION: This radio warning device has a transmitter 2, a receiver 7, a receiving level determining means 3 for calculating a receiving level value on the basis of a plurality of pieces of receiving level information and a distance estimating means 4 for estimating the distance between the transmitter 2 and the receiver 7 on the basis of the receiving level determined by the receiving level determining means 3, wherein the transmitter 2 and the receiver 7 periodically or unperiodically transmits and receives radio to thereby confirm a distance between them, and when the distance estimated by the distance estimating means 4 is equal to or larger than the predetermined distance, functions of a main body device connected to the transmitter 2 or the receiver 7 are restricted or a warning is sent. COPYRIGHT: (C)2007,JPO&INPIT