The present invention provides a device for measuring a mean free path capable of measuring directly the mean free path of a charged particle, a vacuum gauge, and a method for measuring a mean free path. The device for measuring a mean free path according to one embodiment of the invention includes an ion source for generating an ion, a collector (24a) for detecting the number of first charged particles being charged particles having a first flight distance L1 that is a flight distance of zero or more from the ion source, and a collector (24b) for detecting the number of second charged particles having a second flight distance longer than the first flight distance. The control part of the device calculates the mean free path from a ratio between the numbers of the first and second charged particles.