Device for measuring mean free path, vacuum gauge, and method for measuring mean free path

A device comprising:

• a generation source for generating the charged particle;
• a detection means having a first detector arranged at a position for detecting a number of first charged particles of charged particles having flied a first flight distance that is a flight distance of 0 or more from the generation source and a second detector arranged at a position for detecting a number of second charged particles of charged particles having flied a second flight distance longer than the first flight distance;
• and
• a calculation means for calculating the mean free path from a ratio between the numbers of the first and second charged particles,
• wherein the device is configured so that both charged particles flying the first and second flight distances which are generated from the generation source pass through an atmosphere gas to be measured to be detected by the detection means.