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; if yes, returning to said Step (E); and if no, outputting said warning signal to alert said driver.