The present disclosure relates to a sensor module for measuring a displacement occurring in a sensor by a confocal principle, a strain sensor device comprising the same, and a method for measuring a strain in a target using the same. Specifically, the sensor module according to an embodiment of the present disclosure includes a first single-mode optical fiber, a first GRIN optical fiber, a multi-mode optical fiber, a second GRIN optical fiber and a second single-mode optical fiber connected in an axial direction, wherein light inputted through the first single-mode optical fiber is transmitted to the second single-mode optical fiber through the series of optical fibers, and light forming a focal point in the core of the second single-mode optical fiber is detected using a confocal principle.