This study elucidated the following two points concerning the geographic variations of sei-daku opposition. First, nasalization of b and/or z always implies that of d and g. Second, voicing of postvocalic seion and nasalization of postvocalic dakuon are lost more rapidly in p:b and s:z oppositions, in which seion are now basically voiceless fricatives, than in t:d and k:g oppositions, in which seion can be realized as voiced stops.