An improved version of the laser speckle strain gauge is presented in which speckle displacement caused by deformation of a laser-illuminated object is detected in real time using new photodetectors in place of the linear image sensors and microcomputer in the conventional version. The detector, called a spatial filtering detector with an electronic scanning facility, produces a voltage directly proportional to speckle displacement. Therefore, this strain gauge, which utilizes differential speckle displacement, delivers a voltage that is proportional to surface strain. It has a strain sensitivity of a few millivolts per microstrain. We applied this gauge to strain measurements of high-polymer films in various directions under loading at frequencies up to several tens of hertz and were able to evaluate their Poisson ratios.