Several instruments have hitherto been deviced for use to measure the stress in rock or concrete constructions. The authors have, however, attempted to develop a new instrument for the same purpose based on photoelasticity. This instrument has proved suitable for the measurement of stress variation over a long period of time, though it is not available for remote measurement.The principle to determine stress by this instrument is as follows: A glass gage is fixed tightly in a cavity on the surface of a body whose stress variation is to be measured. When there takes place any variance of stress state in the body, there appears some stress in the gage, which is determined by a polariscope. There is a definite relation between the stress variation in the body and the stress that occurred in the gage. Therefore we can determine the former from the latter.Three kinds of gages have been designed and subjected to test, and it has been found that a hollow cylindrical gage is best suited.The polariscope designed for use in this stress measurement is of portable reflection type, about 500gr in weight, furnished with a detachable compensator. For the light source, the white light from a miniature bulb is used.The stress state in a hollow cylindrical gage caused by the change in stress in the body has been analyzed by the theory of elasticity. Assume that a gage was fixed to a body which was then free of stress but now be in a two-dimensional stress state with p and q as the principal stresses, it will now be easy to realize that the gage is in a complicated state of stress. From the results of analysis, the contours of equal values of principal stress difference have been determined and illustrated in Fig. 4, assuming that the ratio of the outer to the inner radius of the gage be 6:1, the Young's moduli of the body and the gage be respectively 2.1×105 and 6.3×105kg/cm2 and the Poisson's ratios of both be 0.2. It is readily noticed from these figures that the stress pattern depends upon the ratio as well as the magnitudes of p and q. Further analysis has shown that the Poisson's ratio of the body affects a little of the stress in the gage.From these results of analysis the most convenient way to find p and q has been studied to obtain the following method:From the shape of stress pattern the directions as well as the ratio of p and q are determined by consulting with Figs. 5 and 6. The magnitude of q can be found from the color of the isochromatics which pass certain definite points. The determination of p and q can also be conducted by manipulating the compensator installed on the polariscope.At the end of analysis, assuming that a gage was fixed to a body which was already in a certain state of stress, the relation among the initial and final stress states and the measured stress has been discussed.Long experience in using this instrument for determining stress variation in rock has shown that it has several advantages, e.g. it is suitable for measurement over a long period of time; it enables us to determine p and q by observing only one gage; the elastic constants of the body have less influence upon the results and the record of measurement can be kept as a color photograph.