This paper describes accenting characteristics of Japanese loanwords, and proposes a strategy to determine accent types of these words. About four hundred fifty unitary loanwords are prepared having more than 5 morae. Six native speakers, with a Tokyo dialect, are instructed to assign the most separable intra-word juncture and accent location of the words. The experimental result shows that Japanese loanword accentuation is closely related to the word formation quasi-structure and the metrical structure formed by compounded syllables in the word. The syllable compounding indicates that Japanese loanwords are represented by sequences of syllable complexes, and a phonological structure can be given to a loanword without morphological structure. Thus, it is clarified that accentuation rules parallel Japanese compound word accent rules. Moreover, an additional new accenting rule is also proposed based on the Japanese metrical structure of unitary words. The newly obtained relation between syllable compounding and accentuation will successfully explain not only the previous rule that the accent is placed at the antepenultimate mora, but also general accenting characteristics including exceptions to the rule.