Disclosed are a Mach-Zehnder interferometric optical modulator and a method for manufacturing the same. The modulator includes first and second lower clad layers stacked on a substrate, a core layer on the first and second lower clad layers, a first upper clad layer on the core layer, a second upper clad layer on the first upper clad layer, and electrodes on the second upper clad layer. The second upper clad layer includes an input waveguide, an output waveguide spaced apart from the input waveguide, branch waveguides branched from the input waveguide and coupled to the output waveguide, and insulating blocks provided on both outer sides of the branch waveguides.