An optical amplifier includes: a first optical fiber, through which seed light and excitation light propagate; an optical coupler that inputs the excitation light into the first optical fiber; a first lens to which the seed light and the excitation light output from the first optical fiber are input and which increases diameters of the seed light and the excitation light; a glass rod doped with rare earth elements to be excited by the excitation light, to which the seed light and the excitation light output from the first lens are input and which amplifies and outputs the seed light as output light; a second lens to which at least the output light output from the glass rod is input and which decreases a diameter of the output light; and a second optical fiber to which the output light output from the second lens is input.