To provide a lost child prevention system to prevent a child from being lost without necessitating difficult operation and technology.

The lost child prevention system 10 is constituted of a master device 1 with a mutual communication function by radio and slave devices 2, 3, 4.... Distance between the slave devices 2, 3, 4... and the master device 1 is measured by a distance measuring means 1b at the master device 1. In addition, an alarm is rung when the distance between the slave devices 2, 3, 4... and the master device 1 which is measured by the distance measuring means 1b becomes equal to or more than preset normal distance by arm alarming means 1c. Thus, the child is prevented from being lost without necessitating the difficult operation and technology since the alarm is rang when the child is apart from parents at equal to or more than the normal distance when the normal distance is preset, the parents have the master devices 1 and the children have the slave devices 2, 3, 4... in the lost child prevention system 10.