In this study, we examine the process of informal learning in the workplace by observing the members who perform trading activities in Company K. By applying Engestrom’s(1987) ‘Activity Theory’ as a framework, we explored the introspective informal learning acquired through contradictory relations within interaction between activity factors. Such ethnographic methods as participatory observation, ethnographic interviews, informal interviews, and data analysis were adopted for this study and the results were as follows. In a workplace environment, we formed a collaborative or contradictory relation between subjects and their interaction with elements of activity. Through this relation, the subject was involved in informal learning by conversing, observing, collaborating, consultation with experts, and introspecting on their experience acquired through expectation, emotion, and opportunity. The contradiction between activity systems was not formed through the interaction but in the course of developing into another system. During this process, the subjects became the transformative subject of introspection. The transformative subject was defined as the subject that continued to grow from the previous system of activity to the new one through interaction with the elements of activity.