The present invention discloses an automatic page detector, to determine which page of a book is open. The automatic page detector uses a magnet and an inductor as a sensor. In the invention, there is a magnet at each page of the book and the locations of the magnets for different pages are different. There is an array of inductors just beneath the magnets when the book is closed. The inductors are connected to the feedback loop of a LC oscillator through analog switches. The proximity of a magnet to an inductor will change the frequency of the LC oscillator. Scanning the analog switches by a microprocessor and detecting the variation of frequency of the LC oscillator during each scanning time period, the status of each magnet will be detected and we can determine which page of a book is opened.