A small amount of birefringence caused by the photoelastic effect from residual strains, crystal- defects-induced strains, and process-induced strains in Si wafers has been measured by using an improved version of a scanning infrared polariscope (SIRP). The SIRP presented here has high sensitivity sufficient to detect the small amount of strain induced near the wafer-supporting finger by the wafer weight itself. It is found that an anomalous amount of strain is induced by slip-line generation during the thermal process and also that a concentric ring pattern of strain is induced by OSF rings. From these results, it is suggested that SIRP is very useful for Si wafer inspection and Si process evaluation in various phases.