PURPOSE: To obtain an optical rain gage which can measure the radius of a rain drop and the amount of rainfall with high precision in a short time.

CONSTITUTION: When distances of luminescent spots formed by a reflected light and a transmitted refracted light from a rain drop 1, from the center of the rain drop on a photosensor 6, are denoted by r1 and r2, angles of incidence of the reflected light and the transmitted refracted light on the rain drop 1 by θ1 and θ2, the magnification of an imaging optical system 5 by (m) and the radius of the rain drop 1 by R, an arithmetic device 8 determines R by an equation R=(r1+r2)/m.(sinθ1+sinθ2). When a distance between the object- side principal point of the imaging optical system 5 and the rain drop 1 is denoted by H, an angle formed by the optical axis of images of first luminescent spots in a pair and by a reference line by δ1 and an angle formed by the optical axis of images of second luminescent spots in a pair and by the reference line by δ2, the amount ΔH of deviation of the rain drop 1 from a focal surface is computed from dimensions of these δ1 and δ2 and a corrective value R' of R is determined by an equation R'=R×(H+ΔH)/H.