This paper presents an agent-based simulator to investigating customer walking flows and purchasing behaviors in a super market. So far, such investigations have cost very much to examine in real situations. The simulator enables us to carry out “virtual experiments” through changing various parameters of retail businesses and store operations. For this purpose, first we observe an actual retail store and analyze sales data. Then we develop the simulation model: Agent-Based In-Store Simulator (ABISS). Intensive experiments have revealed that the flow of customers, which is related to the sales, depends on the design of a store and that the places of in-store advertisement and recommendation system vary their sales.