A redundant amplifier (2, 10), comprising: a first switch for performing switching of connecting, on a one-to-one basis, m inputs P1, P2,..., and Pm to m of n outputs Q1, Q2,..., and Qn, where m and n are natural numbers and m<n is satisfied; a second switch for performing switching of connecting, on a one-to-one basis, m of n inputs R1, R2,..., and Rn to m outputs S1, S2,..., and Sm; and a plurality of n same amplifiers A1, A2,..., and An connected on a one-to-one basis between the n outputs Q1, Q2,..., and Qn of the first switch and the n inputs R1, R2,..., and Rn of the second switch, wherein signal paths L1, L2,..., and Lm are formed in accordance with a connection state between an input and an output of each of the first switch and the second switch, the signal paths L1, L2,..., and Lm connecting the input P1 and the output S1, the input P2 and the output S2,..., and the input Pm and the output Sm, respectively, via any one of the n same amplifiers A1, A2,..., and An, and wherein the connection state has at least two types in which the signal paths L1, L2,..., and Lm each have the same length, wherein the first switch comprises first waveguide switches (3c, 3e) and second waveguide switches (3b, 3d, and 3f), each of the first waveguide switches (3c, 3e) connecting between the input and one of the second waveguide switches (3b, 3d, and 3f); and each of the second waveguide switches (3b, 3d, and 3f) connecting one of the first waveguide switches (3c, 3e) to an amplifier of the plurality of amplifiers, and wherein the second switch comprises third waveguide switches (5c, 5e) and fourth waveguide switches (5b, 5d, and 5f), each of the third waveguide switches (5c, 5e) connecting between one of the fourth waveguide switches (5b, 5d, and 5f) to the output; and each of the fourth waveguide switches (5b, 5d, and 5f) connecting between an amplifier of the plurality of amplifiers to one of third waveguide switches (5c, 5e), wherein the amplifiers A1, A2,..., and An include m active system amplifiers and (n-m) redundant system amplifiers, wherein in one of the connection states, each of the signal paths L1, L2,..., and Lm includes one of the m active system amplifiers and all lengths of the signal paths L1, L2,....and Lm are equal to each other, wherein in another of the connection states, at least one of the signal paths L1, L2,..., and Lm includes one of the (n-m) redundant system amplifiers and each of the other signal paths includes one of the m active system amplifiers and all lengths of the signal paths L1, L2,..., and Lm are equal to each other. a first switch for performing switching of connecting, on a one-to-one basis, m inputs P1, P2,..., and Pm to m of n outputs Q1, Q2,..., and Qn, where m and n are natural numbers and m<n is satisfied; a second switch for performing switching of connecting, on a one-to-one basis, m of n inputs R1, R2,..., and Rn to m outputs S1, S2,..., and Sm; and a plurality of n same amplifiers A1, A2,..., and An connected on a one-to-one basis between the n outputs Q1, Q2,..., and Qn of the first switch and the n inputs R1, R2,..., and Rn of the second switch, wherein signal paths L1, L2,..., and Lm are formed in accordance with a connection state between an input and an output of each of the first switch and the second switch, the signal paths L1, L2,..., and Lm connecting the input P1 and the output S1, the input P2 and the output S2,..., and the input Pm and the output Sm, respectively, via any one of the n same amplifiers A1, A2,..., and An, and wherein the connection state has at least two types in which the signal paths L1, L2,..., and Lm each have the same length, wherein the first switch comprises first waveguide switches (3c, 3e) and second waveguide switches (3b, 3d, and 3f), each of the first waveguide switches (3c, 3e) connecting between the input and one of the second waveguide switches (3b, 3d, and 3f); and each of the second waveguide switches (3b, 3d, and 3f) connecting one of the first waveguide switches (3c, 3e) to an amplifier of the plurality of amplifiers, and wherein the second switch comprises third waveguide switches (5c, 5e) and fourth waveguide switches (5b, 5d, and 5f), each of the third waveguide switches (5c, 5e) connecting between one of the fourth waveguide switches (5b, 5d, and 5f) to the output; and each of the fourth waveguide switches (5b, 5d, and 5f) connecting between an amplifier of the plurality of amplifiers to one of third waveguide switches (5c, 5e), wherein the amplifiers A1, A2,..., and An include m active system amplifiers and (n-m) redundant system amplifiers, wherein in one of the connection states, each of the signal paths L1, L2,..., and Lm includes one of the m active system amplifiers and all lengths of the signal paths L1, L2,....and Lm are equal to each other, wherein in another of the connection states, at least one of the signal paths L1, L2,..., and Lm includes one of the (n-m) redundant system amplifiers and each of the other signal paths includes one of the m active system amplifiers and all lengths of the signal paths L1, L2,..., and Lm are equal to each other.