A laser system including a controller for monitoring and controlling various functions of a laser assembly. 5 or more and a light transmittance at the emission peak wavelength of the semiconductor light emitting element of less than 20%. The resulting polymer does not contain any photoinitiator and is therefore useful as a coating or packaging material for foods, or as a coating or packaging material for any application where the exclusion of photoinitiator, and especially residual photoinitiator, is preferred, such as an outdoor weatherable application. The power unit can be miniaturized by suppressing the projection of the hydraulic cylinder from the engine body in the axial direction. The rubbing apparatus, the pre-treatment apparatus and the light irradiation apparatus are constructed in an in-line type. This invention thus relates to a 3,4-dihydroxyhydrocinnamic acid derivative useful as an anti-diabetic agent, represented by the following formula I:or pharmaceutically acceptable salts thereof, to a method for preparing the compound, and to a composition for treating diabetes comprising the same as an active ingredient.