An optical splitter includes a main waveguide, a tapered waveguide, and a split waveguide group having first and second split waveguides. The side portions of the tapered waveguide taper and expand from a first end portion toward second end portions. The widths of the split waveguides differ from each other. The whole width of the split waveguide group at connecting portions between the tapered waveguide and the split waveguides is smaller than the width of the tapered waveguide at the second end portions. Step portions are formed between the split waveguide group and the side portions.