A planar waveguide optical amplifier includes a substrate and an active waveguide formed on the substrate for imparting gain to an optical signal propagating therethrough. The apparatus has a light dividing member for dividing linearly polarized light beams superposed one upon another into a plurality of light beams. Specifically, during the epitaxial growth of the GaN crystal, the GaN crystal is doped with Mg or Zn in an atmosphere of hydrogen at a temperature of 600 C. The first distance is longest in a length direction. The compounds have gastric secretion inhibiting and excellent gastric and intestinal protective action properties.