This paper proposes a new model that measures customer loyalty from the days taken to repurchase in a shop. This model is based on IRT (Item Response Theory) and geometric distribution. It enables us not only to estimate customer loyalty as latent trait variables of each customer, but also to quantify the ability to pull in customers of shops. Moreover these shops could be any selling segment, for example, brand stores, manufacturers, or articles. In order to check the validity of this model, it is applied to actual ID-POS (point-of-sale with identification) data. In addition, another model using exponential distribution is proposed as an alternative model.