To provide a instrument for measuring fluid film thickness which is easy to adjust and has high resolving power both in the horizontal and vertical directions, and to provide a fluid film thickness measuring method.

This instrument comprises a sealing member 11, a mating member 12 having light transmissivity, a fluid 13 including a fluorescent substance and sealed up by the contact of the member 11 with the member 12, a light source 14 for emitting a laser beam 14a, a scanning means 15 for scanning with the beam 14a, a photographing device 16 for photographing a fluorescent image 20, a beam splitter 17 for guiding the beam 14a from the light source 14 to a measured area and joining the image 20 emitted from the fluorescent substance with the photographing device 16, a microscope 19 for condensing the beam 14a guided by the beam splitter 17 to the measured are in the fluid 13 and magnifying the image 20 emitted from the fluorescent substance to guide it to the beam splitter 17, and an optical filter (not illustrated) which is disposed in from of the imaging device 16 to remove light of a specific wavelength from the beam 14a.