Helen and GUEST Q, thank you very much. I feel we are one step coming close to it. A hymn tune seems to be a good suggestion, because some hymn tunes (such as "What a Friend We Have in Jesus") were adopted in school songbooks at that time, "disguised" as secular songs with completely different non-religious words. I'll look into Japanese hymnals published in those days.

I saw an edition (Meiji Shooka Bassui Shoogaku Shooka, 1895, pp. 42-43) containing this song at Digital Library from the Meiji Era (Japanese only) HERE (National Diet Library of Japan), where there is no mention of its source at all.

BELMONT is the tune of "The Spirit breathes upon the Word" (William Cowper) in Japanese hymnals: Sanbika Dai-Ni-hen (1974, No. 34) and Sanbika 21 (1997, No. 54), with different translations. There seems to be no one here in Japan who suggests the relation between the two tunes.

~Masato