This study develops a flexible artificial hand system with slip sensors. The artificial hand is composed of four flexible actuators. Two pairs of two opposite actuators are driven by the independent air pressure sources each. In each pair, one of opposite pair of actuators is equipped with a slip sensor. The slip sensor is composed of a photo reflector and a conductive rubber sensitive to load force. The contact of the hand to an object is detected by the change of resistance of the conductive rubber. After the detection of contact, the slip velocity is measured by calculating the cross-correlation function of the output voltage waveforms of two photo transistors in the photo reflector. The grasping force of the hand is increased according to the measured slip velocity. The calculation of the cross-correlation function and the generation of control signals are made in a host personal computer. From the experimental results of holding several objects, the hand system is proved to grasp suitably the object whose weight is previously unknown.