Manipulating objects whose physical properties are unknown remains one of the greatest challenges in robotics. Controlling grasp force is an essential aspect of handling unknown objects without slipping or crushing them. Although extensive research has been carried out on grasp force control, unknown object manipulation is still difficult because conventional approaches assume that object properties (mass, center of gravity, friction coefficient, etc.) are known for grasp force control. One of the approaches to address this issue is incipient slip detection. However, there has been few detailed investigations of robust detection and control of incipient slip on rotational case. This study makes contributions on deriving the theoretical model of incipient slip and proposes a new algorithm to detect incipient slip. Additionally, a novel sensor configuration and a grasp force control algorithm based on the derived theoretical model are proposed. Finally, the proposed algorithm is evaluated by grasping objects with different weights and moments including a fragile pastry (éclair).