A strain measuring device is provided which is not affected by a change in the intensity and irradiation direction of light received by a measurement target and which enables stable measurement. A computer functions as minute region extracting device for extracting respective surface height distributions of minute regions a and b containing points A and B in a predetermined region from an initial surface height distribution obtained by measuring the predetermined region (6) of the measurement target by a surface height measuring device, coordinate calculating device for calculating coordinates of points A′ and B′ in minute regions a′ and b′ most similar to the minute regions a and b over a time-advanced surface height distribution of the predetermined region 6 and corresponding to the points A and B in the minute regions a and b, respectively, and strain calculating device for calculating a strain in a direction of a line AB of the measurement target.