This paper discusses the practice of lay Buddhists' faith in Theravada Buddhism based on the literature of Upāsakajanālaṅkāra, written by Ānanda, a Buddhist monk of the Theravada tradition from around the 12th century. As is already well known, a Buddhist order consists of the fourfold assembly: bhikkhu, bhikkhunī, upāsaka, upāsikā. Although their roles are different due to differences in terms of a monk or a lay person, each of them have importance as the four pillars that support the Buddhist order. As the extant early Buddhist literature has been handed down by monastics, the teachings have mostly been about and directed to monastics. As a result, there have been many studies conducted and accumulated on the disciplines and lives of monastics. On the other hand, few studies have been conducted on the practice of lay Buddhists due to the lack of materials or indifference. Thus, more studies on lay Buddhists from various perspectives are required in the future, considering their importance as a member of the fourfold assembly. Upāsakajanālaṅkāra is a book that teaches how to practice the way to Buddhahood for upāsaka, as indicated in the book title. Although this book was written in the 12th century and relatively late, the theme is presented based on the Pāli tipiṭaka 三藏, which follows the basic ideas of Theravada Buddhism transmitted since early Buddhism. This book also explains how to perform the practice of lay Buddhists' faith under a two-fold structure: “right behavior based on the Principle of the Way or Truth” and “religious practice based on right behavior.” That is, once right faith is established through the three refuges 三歸依, then ethical life can be achieved through right behavior or following the five basic precepts 五戒. Furthermore, this book suggests how lay buddhists can practice their faith by building 10 puññakiriyavatthu 福業事 and removing obstructions to attaining enlightenment and Nirvāṇa, thereby practicing religious teachings that lead to happiness in the mundane world and super-mundane world. Many people who follow the way to Buddhahood seek an easy and convenient life and abandon the right lifestyles that Buddhists should follow. However, the practice of lay Buddhists' faith are also as sophisticated and systematic as those of monks and nuns. Just as monastics devote their lives to supreme goodness and virtue through their lives, lay Buddhists should also strive for practice and faith that lives up to the way to Buddhahood, and endeavor to live right lives that are worthy of the name of Buddhists. In this way, the understanding of the practice of faith for upāsaka presented by Upāsakajanālaṅkāra is urgently needed to establish right lives that modern lay Buddhists should follow.