PROBLEM TO BE SOLVED: To produce a fine powdery nutrient supplementary preparation of good quality made of a bone of an eel or a conger as a raw material by enabling the removal of a bone marrow liquid from the bone of the eel, or the like, which is conventionally considered as difficult. SOLUTION: A bone (a) taken out of an eel, or the like, is subjected to primary washing, mincing, secondary washing, or the like, to provide a rice grainy bone grain (b), which is then drained and placed in a retort pouch to remove a bone marrow liquid from the bone grain (b) by a high-pressure and high-temperature treating step 5a. The resultant bone grain (c) is then passed through a tertiary washing step 5b, a hot-air drying step 6 and a pulverizing step 7 to afford a powder (d), which is subjected to a sterilizing and deodorizing step 8 for irradiation of infrared rays for a prescribed time. Thereby, the powder becomes suitable for drinking and the obtained powder is finally sieved in a particle grading step 9 to produce a fine powder (e) as a nutrient supplementary preparation having =200 mesh particle size.
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