To provide a key agreement method, with respect to encryption by common share of a secret key and, especially for a secured communication system in a multiplex access system.

The key agreement method includes a step (step 500), where a 1st user device encodes a signal from a signal source by a bit sequence and transmits the encoded signal, a step (step 502) where a 2nd user device being a regular communication opposite party of the 1st user device decodes the signal transmitted from the 1st user device to measure the decoded signal, a step (step 504) the 2nd user device selects only bits, having a measured value that is a predetermined threshold value or over, a step (step 506), where the 2nd user device informs the 1st user device about only a number denoting to which order number bit in the bit sequence the adopted bit corresponds in place of the adopted bit value, and a step (step 508), where the 1st user device and the 2nd user device commonly share the adopted bit as a key string and abort the remaining it.