This study aimed to develop and validate a scale to measure the Grammatical Carefulness (GC) of foreign language learners. GC, by its definition, refers to psychological, behavioral, and meta-cognitive traits of a learner, and it entails highly controlled, cautious, analytical, and time-consuming language use. By conducting a set of questionnaire surveys targeting Japanese junior high school, high school, and university students (N = 2,288), a Foreign Language Grammatical Carefulness Scale (FLGCS) with 14 items, written in Japanese, was developed and tested for its factorial structure, reliability, convergent, content, and criterion validity. The results demonstrated that FLGCS yields three factors: (a)phonological, (b) lexical-syntactic, and (c) pragmatic carefulness, with a high reliability for each. The factorial validity was also supported by using both xploratory and confirmatory factor analyses. Further, a set of analyses confirmed various types of validity. The evidence for the validity is as follows: (a) the linguistic experts (n = 10) consistently judged that all the items properly referred to each factor in an appropriate linguistic sense, (b) FLGCS showed correlations with learner beliefs, consistent with theoretical expectations, and (c) FLGCS correlated to the scores of a C-test, and with the time to finish the C-test. The applicability of FLGCS in EFL teaching and research will also be discussed.