The benefit of Content and Language Integrated Learning (CLIL) for second language (L2) development has been addressed by second language acquisition theory and investigated by empirical studies. However, previous studies have not demonstrated the effectiveness of CLIL precisely as most of their study participants took CLIL and non-CLIL courses concurrently. Additionally, existing studies have focused on the effectiveness of CLIL in primary and secondary education and rarely addressed it in higher education. The present study considered the limitations of the previous studies and investigated the development of L2 English productive skills (i.e., speaking and writing) through CLIL conducted in a university in Japan. This study employed a pre-experimental pretest-posttest design, in which all participants received CLIL instruction for four months undertaking the pretest and posttest. The results showed that their productive skills improved significantly. The analysis of pretest and posttest scores also revealed the specific areas of productive skills that benefitted from CLIL, such as Task Achievement (speaking and writing), Coherence and Cohesion (writing), and Grammatical Range and Accuracy (writing). However, this study revealed that the degree of improvement was small, and that vocabulary and critical thinking skills did not improve significantly. This study’s findings provide empirical evidence for the effectiveness of CLIL itself on L2 development in higher education.