Japanese speakers who can communicate appropriately with sociolinguistic competence are able to shift between formal and plain styles and use honorifics in a single conversation with the same speaker. In this paper, second language (L2) and first language (L1) speakers' instances of style-shifting in dyadic conversations with a teacher are examined quantitatively by analyzing forms, and qualitatively by exploring L1 listeners' attitudes. Statistical analysis of L2 and L1 speakers' style-shifting showed that L2 speakers tended to use formal styles in statements and verbs, whereas L1 speakers tended to use them in questions, nouns, and main clauses. Qualitative analysis of the interview responses suggests that evaluation of L2 speakers' speech and politeness is multifaceted and mixed, showing both positive and negative reactions. The results of this study have implications for acquiring sociolinguistic competence of style-shifting.