Frozen fillet or slice of fish meat, or a frozen processed fillet or slice of fish meat, in which degeneration due to TMAO is inhibited or prevented during freeze-storage

Provided is a frozen fish fillet or slice, or a frozen processed fillet or slice of fish meat preferably prepared from white-meat fish in which degeneration due to the decomposition of trimethylamine-N-oxide (TMAO) which would otherwise occur during freeze-storage is inhibited, which is brought about by subjecting the fillet or slice to water immersion treatment and/or oxidation treatment prior to freeze-storage. Specifically, inhibition of the decomposition of TMAO in fish fillet or slice during freeze-storage is achieved by transferring the fillet or slice to an oxygen-packed container, subjecting the meat preparation to oxygen substitution treatment, or exposing the meat preparation to an oxidant-containing solution, prior to freeze-storage.