A method is proposed to control human grasping force unconsciously by generating small distributed vibration stimuli on a finger based on a human reflex action against detection of partial slippage of contact. Human beings can increase grasping force before the grasped object slips off the fingers based on afferent signals from tactile receptors that detect the expansion of the partial slippage area. We assumed that vibration stimuli that have an appropriate frequency and distribution generate a virtual sensation of partial slippages and induce a reflective grasping force increase. Grasping experiments with minute distributed vibration stimuli using ICPF (Ionic Conducting Polymer gel Film) actuators were performed. Experiments results showed that the effective vibratory frequency was 30 Hz, which is the most sensitive range for Meissner's corpuscles that are closely related to detection of partial slippage. We also found that distribution of vibration stimuli were important to produce human grasping force increase.