This paper proposes a method of estimating contact force between environment and an object grasped by a parallel two-fingered hand with tactile sensors developed by us. The sensors can get 2D patterns of pressure distribution and have many potential applications such as recognition of a grasped object, assembly, tasks and so on. In order to achieve assembly tasks using a multi-fingered hands, next two kinds of parameters are essential: one is the geometrical relationship between the coordinate systems of the hand and of the grasped object, and the other is the information about the position of contact point and the contact force between the grasped object and its environment. When the parallel two-fingered hand are used for assembly tasks, the geometrical relationship between the hand and the object can be easily obtained. Thus, this paper describes how to detect the contact point and to measure the contact forces by the tactile sensors. First, the principle of the sensor is explained briefly. Second the way to use the tactile sensor as a force sensor is shown. Since the information front the sensor is not enough to identify, the contact point and the contact force, additional measurements at different positions are needed. The algorithms for the tactual behaviors are proposed.