Humans can change the grasping force before the grasped object slips off the fingers unconsciously. This reflex is called inducing reflective grasping force control. This paper describes the method of inducing reflective grasping force control and the observation result. To clarify the conditions of stimuli for inducing reflective grasping force control, we developed the observation equipment with piezoelectric actuators and force sensor. Vibration stimuli and force senses are presented to a human finger, and change of the grasp force are observed by the force sensor. In this paper, we change the parameters of the vibration stimuli and force senses and detect the difference of grasp force.