Abstract: An optical device including first and second Mach-Zehnder type optical interference units cascaded with each other. The first Mach-Zehnder type optical interference unit includes a pair of interference optical waveguide arms having different lengths adjusted so that the phase difference of propagated light becomes &pgr;. A first heater is mounted on the longer optical waveguide arm of the first interference unit. The second Mach-Zehnder type optical interference unit includes a pair of interference optical waveguide arms having equal lengths. A second heater is mounted on one of the optical waveguide arms of the second interference unit.