Second-language (L2) knowledge of English is an essential communication tool in the contemporary era of globalization in many research and business fields. Among the important tasks in English education is the measurement of student proficiency. While various tests purport to measure proficiency in learning English, repeated preparation and implementation of tests remain major costs in providing English education. We propose that measurement with an electroencephalogram (EEG) might be used for measuring proficiency in L2 learning of English without the repeated preparation and implementation of English tests. In this study, we proposed the use of an EEG as an index of the frontal theta band activity involved in concentration with the use of English materials without the need for the repeated preparation and implementation of English tests. Our experiments recorded the EEG signals of 52 participants, divided into two groups of those with beginner and advanced knowledge of English. All participants were asked to attend three sessions: before, during, and after the English lesson and the frontal theta band activities were analyzed as index of concentration for these lessons. The results showed that the frontal theta band activity for advanced students during the English lesson was larger than that for beginner students. Frontal brain activity reflects a common computation recognizing the need for cognitive control, such as concentration on L2 learning. Without dependence on existing tests, this study proposes a method to measure L2 proficiency more quickly than the methods used in previous neuroscience studies.