A thin film opto-electric transformer consisting of a p-n junction photo-cell (500×500μm²) and a multilayer spiral coil transformer (1.4×1.4 mm²) monolithically fabricated on a Si substrate is proposed and its fundamental characteristics are demonstrated. The optical power from an LD is converted into electric power by the photo-cell, and then the photocurrent is converted into output voltage V_o by the miniaturized transformer. In this experiment each of the primary and the secondary coils has double layers of rectangular spiral coil and has the same turns of 20 (i.e., 1 : 1 transformer). Frequency dependence of the inductance L of the double layer coil with an outer core is measured. Annealed magnetic film has relatively better characteristics for inductance than non-annealed one. The circuits simulations are performed by PSpice simulator using experimental values of parameters, and the experimental results can be well explained by these simulations.