To adaptively switch a modulation mode according to an operation form of quality-priority and capacity-priority in a multi-value applicable radio device having an active/standby configuration. The comparison unit 28a compares a reception monitoring signal c1 of an active transmission line with a lower-limit threshold LS and an upper-limit threshold HS, while the comparison unit 28b compares a reception monitoring signal c2 of a standby transmission line with the lower-limit threshold and the upper-limit threshold. Then, they output their respective results to a determination unit 28c. The determination unit 28c has therein a low-level flag and a high-level flag that are set/reset based on comparison results in the comparison units 28a and 28b. Conditions for the set/reset are different depending on the operation form. The determination unit 28c determines a modulation mode based on comparison results in the comparison units 28a and 28b and the states of the low-level flags and high-level flags, and outputs modulation mode switching control information e1 and e2.