We measured the intramolecular distribution of 15N as well as conventional nitrogen and oxygen isotope ratios in oceanic nitrous oxide (N2O) in the western North Pacific for the first time. In contrast to a nearly homogeneous vertical distribution of a bulk nitrogen isotope ratio, a wide variation in site preference for intramolecular 15N distribution was found, suggesting the subsurface and deep source mixing and the production mechanism in the ocean. The oceanic composition of N2O isotopomers quantitatively outlines the ocean as the most significant source after the terrestrial one.