1. A method for monitoring a driver comprising steps of: Step (A): capturing an inner cabin image of a cabin; Step (B): performing a face detection of the driver on said inner cabin image to obtain a face detection result; Step (C): storing said inner cabin image and said face detection result corresponding to said inner cabin image; Step (D): repeating said above-mentioned steps, and when an accumulated number of stored said inner cabin images reaches a predetermined value, executing steps of; Step (E): capturing said inner cabin images at a plurality of adjacent storing time points and corresponding said face detection results thereof; Step (F): determining whether at least one corresponding said inner cabin image catches a face of said driver according to each captured said face detection result; if no, outputting a warning signal to alert said driver; and if yes, executing a step of; Step (G): determining whether said faces of two continuous and corresponding said inner cabin images are shown within a reasonable face region according to each captured said face detection result to define a face region correction process; said face region correction process including taking a positional coordinate of the face, taking said positional coordinate having a highest existing probability, and deciding a position of said reasonable face region; and, (1) if said faces of two continuous and corresponding said inner cabin images are within said reasonable face region, then performing a nose detection of said driver on presently-stored said inner cabin image; obtaining a positional coordinate of a nose of said driver; comparing said positional coordinate of said nose with said reasonable face region to obtain a head deflection angle of said driver to define a nose detection process; determining whether said head deflection angle is lower than a predetermined threshold, and returning to said Step (E); and (2) if said faces of two continuous and corresponding said inner cabin images are not within said reasonable face region, then outputting said warning signal to alert said driver.