The study aims to develop and validate an affordable and generalizable survey design tool to measure pre-service EFL teachers' self-efficacy beliefs about cultural and linguistic diversity. It is significant as there are relatively low number of studies aiming to measure such beliefs in today's changing demographic conditions which are beyond its limits especially in some countries due to the massive flow of migrants. Most of these studies are qualitative which are not cost-effective and flexible. In this study, exploratory sequential mixed method was conducted. Initially, qualitative research design was employed. Findings were used to develop a valid and reliable survey design tool in a second quantitative phase in which data was gathered quantitatively to reduce the number of the variables to a few values representing self-efficacy beliefs and determine the latent components in the scale. Final version of the scale included 20 items across five factors after Exploratory Factor Analysis through principal axis factoring and Confirmatory Factor Analysis through maximum likelihood analysis. Findings supported that a set of 20 items in the final scale form was statistically valid and reliable in measuring pre-service EFL teachers' self-efficacy beliefs about teaching in culturally and linguistically diverse student groups.