In late years, the necessity of high Knudsen number flows, such as highly rarefied gas flows with large mean free path and gaseous flows around nanodevices with small characteristic length have increased significantly, especially in fields of researches on space technology, surface science, fabrication of thin film for semi-conductor and so on. In this paper, we describe mainly the optical diagnostic techniques for the high Knudsen number flows, such as laser induced fluorescence (LIF), resonantly enhanced multiphoton ionization (REMPI) and pressure sensitive molecular film (PSMF), and our experimental results obtained by the use of the techniques, i.e., applications of LIF to visualization of flow field structures including complicated shock wave system and to a measurement of rotational temperature, establishment of a REMPI system and its application to detection of rotational nonequilibrium in highly rarefied gas flows, and development of the suitable PSMF for low density and micro-scale gas flows.