Local light intensity at a defect in a photonic crystal structure is g
reatly enhanced from the input light intensity by the existence of the
photonic state localized around the defect. Enhancement of effective
optical nonlinearity of photonic crystal structures with a defect whic
h has optical nonlinearity was considered theoretically, and studied e
xperimentally. In experiment, intensity dependence of the transmittanc
e was measured, and reduction of the effective saturation intensity by
factor of 20 was observed.