Advances in sensor technology are making vehicles increasingly intelligent, with driving becoming safer and even more comfortable. Advanced safety systems continue to evolve, but there are concerns that when an advanced safety system assists driver operations, the driver may place too much confidence in the safety system, leading to careless driving where the driver releases the steering wheel. The Lane Departure Prevention System can detect impending road departure and control the vehicle to aid the driver in not departing from the lane. This has great potential to reduce the number of road departure accidents, but the effectiveness of this system depends largely on drivers’ receptivity to the system. Effectiveness is determined by how much the driver is permitted to relax the grip on the steering wheel when the system assists driver operations, and how much the system assists driver operations. The purpose of this paper is to study the minimum necessary steering wheel grip conditions during Lane Departure Prevention System operation. This study conducted tests with 22 test subjects using an omnidirectional field of view type 6-axis vehicle simulator, and investigated the relationship between the steering wheel grip state and vehicle behavior when the system is unable to function as intended. The results provided information such as when the system is unable to function as intended while driving along a curve, the response time is faster when the driver is touching the steering wheel than when not touching the steering wheel, and this response time becomes even faster depending on the grip state.