1. A radio device for use in an autonomously established radio network by which radio communication is carried out between a source and a destination, comprising: a threshold determining unit determining a threshold used to detect a strength equal to or more than the minimum strength of radio waves transmitted and received in a stable radio wave environment, based on a plurality of radio waves received from n (n is a positive integer) radio devices; a table producing unit producing a routing table that shows routing information between said source and said destination based on the routing information of routes having a reception signal strength equal to or more than said determined threshold; and a route maintaining unit maintaining said routing information stored in said routing table or storing said routing information in said routing table without introducing the threshold determined by said threshold determining unit if strengths of radio waves are in a state of imbalance in the radio communication space between said radio device and said n radio devices.