This paper aims to formulate the private travel behavior by the activity-based travel model and to valuate the travel-time savings of the private trip. The formulated model has a nested structure, consisting of two models : one-day activity model and weekly activity model, both of which are based on the constrained utility maximization. The former model allocates time and cost in a day under the given available time and income, while the latter model determines place and time of activity in a week by allocating time and cost. The weekly diary travel data is used for estimating the coefficients in the model. By applying the De Serpa's definition of the value of travel time savings to this model, the authors valuate it by using the data of rail-users in the Tokyo Metropolitan Area.