A distributed amplifier comprising: an input-side transmission line; M, where M is an integer not less than 2, amplification circuits (33-1A-33-NA, 33-1B-33-NB), corresponding to an at least two branched system; M output-side transmission lines provided in accordance with the M amplification circuits (34-1A-34-NA, 34-1B-34-NB); and a combination circuit (37A, 37B) configured to combine outputs of the M output-side transmission lines; wherein: the input-side transmission line includes an input-side serial line formed by connecting in series M×N, where N is an integer not less than 2, unit transmission lines each including a same line length, and an input-side terminating resistor; the M amplification circuits each include N amplifiers in N stages; the input of each of the N amplifiers of an i-th amplification circuit of the M amplification circuits is connected to the input-side transmission line, wherein the input of a k-th amplifier of the N amplifiers is connected to the ((k - 1) M + i)-th node of the input-side transmission line, where k is an integer not less than 1 and not more than N and i is an integer not less than 1 and not more than M; each of the M output-side transmission lines includes an output-side serial line including N transmission lines each being connected in series between neighboring outputs of the N amplifiers; each transmission line of the output-side transmission lines includes a line length M times the line length of the unit transmission line; the combination circuit includes a phase adjustment unit configured to match the phase of the output of each of the M output-side serial lines with one another; and characterized in that for each of the M output-side transmission lines, each j+1th transmission line in a respective one of the M output-side transmission lines, with j an integer comprised between 1 and N-1, coupled between an output of the j+1th amplifier and the output of the respective one of the M output-side transmission lines, has a line width twice a line width of the jth transmission line in the respective one of the M output-side transmission lines coupled between an output of an jth amplifier and the output of the respective one of the M output-side transmission lines. an input-side transmission line; M, where M is an integer not less than 2, amplification circuits (33-1A-33-NA, 33-1B-33-NB), corresponding to an at least two branched system; M output-side transmission lines provided in accordance with the M amplification circuits (34-1A-34-NA, 34-1B-34-NB); and a combination circuit (37A, 37B) configured to combine outputs of the M output-side transmission lines; wherein: the input-side transmission line includes an input-side serial line formed by connecting in series M×N, where N is an integer not less than 2, unit transmission lines each including a same line length, and an input-side terminating resistor; the M amplification circuits each include N amplifiers in N stages; the input of each of the N amplifiers of an i-th amplification circuit of the M amplification circuits is connected to the input-side transmission line, wherein the input of a k-th amplifier of the N amplifiers is connected to the ((k - 1) M + i)-th node of the input-side transmission line, where k is an integer not less than 1 and not more than N and i is an integer not less than 1 and not more than M; each of the M output-side transmission lines includes an output-side serial line including N transmission lines each being connected in series between neighboring outputs of the N amplifiers; each transmission line of the output-side transmission lines includes a line length M times the line length of the unit transmission line; the combination circuit includes a phase adjustment unit configured to match the phase of the output of each of the M output-side serial lines with one another; and characterized in that for each of the M output-side transmission lines, each j+1th transmission line in a respective one of the M output-side transmission lines, with j an integer comprised between 1 and N-1, coupled between an output of the j+1th amplifier and the output of the respective one of the M output-side transmission lines, has a line width twice a line width of the jth transmission line in the respective one of the M output-side transmission lines coupled between an output of an jth amplifier and the output of the respective one of the M output-side transmission lines.