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, the radio device comprising: a threshold determining unit for 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 radio devices, said n being a positive integer; a table producing unit for producing a routing table that shows routing information between said source and said destination based on routing information of routes having a reception signal strength equal to or more than said determined threshold; and a route maintaining unit for determining whether strengths of radio waves are in a state of imbalance in a radio communication space between said radio device and said n radio devices based on said strengths of radio waves and the number of radio devices adjacent to the radio device, and for 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 the strengths of radio waves are in the state of imbalance, wherein said threshold determining unit includes: a signal strength detecting unit for carrying out signal strength detection processing to detect a first set of m pairs of signal strengths including m maximum reception signal strengths and m minimum reception signal strengths corresponding to said m maximum reception signal strengths, based on said plurality of received radio waves, said m being an integer equal to or more than a prescribed number; a rearranging unit for carrying out rearranging processing to rearrange said detected first set of m pairs of signal strengths so that said m maximum reception signal strengths are in descending or ascending order, thereby producing a second set of m pairs of signal strengths; and a setting unit for detecting two minimum reception signal strengths corresponding to two adjacent maximum reception signal strengths having the largest difference among the m maximum reception signal strengths in said second set of m pairs of signal strengths and setting said threshold in the range defined as having the detected two minimum reception signal strengths as end values.