An optical fiber distributed temperature sensor has been developed for the purpose of continuous measurements of vertical temperature profile in the ocean. The sensor uses essentially OTDR (Optical Time Domain Reflectometry). The fiber was encapsulated in metallic tube. The fiber with total length of 1000 m was duplicated with the result that the sensor can measure the temperature twice at the same depth, which affords us means to make the sensor more reliable. Comparison measurements in the ocean were conducted using the CTD and XBT as standard instruments during about 15 days, summer 1990, in the Toyama Bay. Temperature differences at successive depth with spatial inverval of lm were analyzed. We found, (1) The proposed temperature correction method for the fiber sensor worked well in the whole 17 cases analyzed. (2) Mean spatial temperature difference between the fiber sensor and CTD decrease about 14 % in average, and about 20 % in the case of XBT when we applied the proposed correction technique. (3) The temperature difference in the case of CTD is 0.64°C in maximum and 0.51°C in average, and these in the case of XBT are 0.99°C and 0.73°C, respectively. These values are nearly the same as them obtained on the laboratory experiments in temperature chamber. We could demonstrate the usefulness of the optical fiber distributed temperature sensor, to monitor the spatial temperature distribution in the ocean.