[en] Deuterium substitution of Rochelle salt (potassium sodium tartrate, RS), which is a representative hydrogen-bonded ferroelectric material, causes the shift of two phase transition and the temperature range of the ferroelectric phase is expanded. The previous studies mainly with X-ray diffraction are briefly reviewed. Single crystals of deuterated Rochelle salt are obtained from the heavy water solution. The crystallized sample is sealed in an aluminium capsule. The neutron diffraction data are obtained with a four-axis neutron diffractometer for two paraelectric phases at -200degC adn 40degC. The results are presented and discussed particularly on the position of hydrogen-bonded hydrogen (deuterium). A disordered models of two paraelectric phases and structural a phase transition model based on an order-disorder type are proposed. (T.H.)