Abstract: A dozing preventive device and its method are disclosed, in which the user does not have to move to a specific place for dozing prevention, and neighboring people are not disturbed. The dozing preventive device is small and is simple in its construction so that the user does not have to use his hand for supporting the device. The dozing preventive device comprises: a contacting portion directly contacting with the teeth of a user; a vibration mechanism for vibrating the contacting portion; a power supply for driving the vibration mechanism; and a housing case for accommodating the vibration mechanism and the power supply as well as for supporting the contacting portion. The dozing preventive device is brought into contact with the teeth of a user so that the vibration is transmitted to the user's brain, thereby preventing the user from sleeping.