Helping young adult Japanese students to learn English as a foreign language is one of the important missions of EFL instructors. To carry out this mission effectively, we need to have a sound theory to guide us. Though there are many existing second language acquisition (SLA) theories, such as Krashen's monitor model (1981), McLaughlin's information processing model (1987), Schumann's acculturation theory (1976), and Cummins' language proficiency model (1979), they fail to offer significant insight into adult EFL learning and instruction. Although developments in adult SLA have grown out of demographic changes in North America, Britain and Australia, the suggested SLA model proposed in this article can be applied to the teaching/learning of English as a foreign language at colleges/universities in Japan. The first part of this paper will endeavor to propose an information processing model of adult SLA which consists of three components: processing orientation hypothesis, external and internal conditions, and learning task analysis. The second part of this paper will discuss the methodological implications involved in using an information processing model of SLA when teaching adult learners following an andragogical approach. By andragogical, we mean a model of education for young adults in which the content and the teaching processes are primarily, but not exclusively, determined by the needs, desires, resources, and experiences of the learners who participate actively in shaping and controlling them (Knowles, 1984).