In order to obtain a transformed rice plant having an improved salinity tolerance level because of its enhanced proline accumulating ability, a P5CS (DELTA1-pyrroline-5-carboxylate (P5C) synthetase) gene of rice or a P5CS gene of Arabidopsis thanliana and the antisense (reverse DNA sequence-containing) gene of a ProDH (Proline dehydrogenase) are introduced into a rice plant by using a genetic engineering technology.