The non-native English-speaking teachers' (NNESTs) beliefs about the monolingual approach have not been sufficiently studied in the teaching of English as a foreign language (EFL). In examining the NNESTs' beliefs about that issue, the present study adapts Guy Cook's recent framework, according to which the monolingual approach is based upon four untested, yet influential assumptions in English teaching (i.e. monolingualism, naturalism, native-speakerism, and absolutism). These theoretically intertwined assumptions lend powerful support to the monolingual approach. The participant questionnaire based on the aforementioned framework was administered to 207 Korean pre-service and in-service teachers of English, along with a post-survey interview with a subset of the participants. In general, the results showed that the in-service teachers were more oriented towards the bilingual approach, but less in agreement with the ideas centring on the monolingual approach, than their pre-service counterparts. It was further found that the in-service teachers' negative attitudes towards the monolingual approach may come from their actual teaching experience, rather than from teacher variables, such as the amount of experience abroad and English proficiency.