In some west Japanese dialects, the quotative marker (e.g. -to) can be omitted in the environment that the verb iu "say" or omou "think" directly follows the quotative phrase. This paper analyzes this phenomenon quantitatively, using corpuses of dialect conversation. As a result of analysis, we found that the frequency of the zero-marking is higher in South-Hyogo, Okayama, and Hiroshima than other areas. Furthermore, we confirm that the frequency is higher in the environment that be followed the verb iu than the verb omou, and higher when -to iu means "say that" than when it means "call something a particular name", or when it becomes a compound functional expression. These internal linguistic factors are found commonly among areally discontinuous dialects. Therefore we come to the conclusion that the frequency of the zero-marking becomes higher when the functional load of the quotative marker is lighter.