The present invention relates to an optical filter comprising a simple structure which easily realizes slope control of loss spectrum in a signal wavelength band as a gain equalizer. The optical filter comprises a first Mach-Zehnder interferometer constituted by a first part of a main optical line, a first auxiliary optical line, and first and second optical couplers which optically couple the main optical line and first auxiliary optical line to each other. The optical filter further comprises a second Mach-Zehnder interferometer constituted by a second part of the main optical line, a second auxiliary optical line, and third and fourth optical couplers which optically couple the main optical line and second auxiliary optical line to each other. In particular, in order for the slope of loss spectrum in a signal wavelength band to be adjustable while being centered about the amount of loss of light at a reference wavelength propagating through the main optical line from its entrance end to exit end, at least one of the first part of main optical line and the first auxiliary optical line is provided with a first temperature regulating device, at least one of the second part of main optical line and the second auxiliary optical line is provided with a second temperature regulating device, and the first and second temperature regulating devices are controlled by a control system.