PURPOSE: To obtain an optical device to be used for processing a new type optical signal by using a Fabry-Perot cavity consisting of a partial waveguide part and a partial non-waveguide part.

CONSTITUTION: The cavity consists of a partial waveguide 43 and a partial non-waveguide gap 52. A small change in the size of the cavity can be attained by a metal ring 41 and a piezoelectric element 49 stuck to a fiber 40 by epoxy pressure sensitive adhesive 50. Center and right fiber elements 43, 44 are fixed in a sleeve so as not to be almost moved and a left element 42 is arranged so as to have a large chance to be moved because the size of the gap 52 is increased and the optical length of the cavity is increased at the time of starting a piezoelectric device. The gap 53 is set up in a part other than an optical cavity part so as to optimize operation. Consequently, the device capable of controlling resonance wavelength by changing the length of the gap 52 or the waveguide 43 or both of the gap 52 and waveguide 43 can be obtained.