There is provided a waveguide type optical device whose parasitic capacitance is reduced to allow an increase in signal transmission speed. Bottom electrode 41 is formed on substrate 2, bottom cladding 51 is formed on bottom electrode 41, and bottom core 62 is formed on bottom cladding 51. Top core 61 is formed on bottom core 62, top cladding 53 is formed on top core 61, and top electrode 42 is formed on top cladding 53. Two sides of top core 61 and bottom core 62 are covered with side cladding layer 52. Vertically overlapping portions of top electrode 42 and bottom electrode 41 are located almost at a same place as a region for a core layer composed of top core 61 and bottom core 62. The width of one from among top core 61 and bottom core 62 is satisfying a single mode condition, and the width of the other is almost equal to or more than the width of a field distribution.