A coin acceptor includes main body having coin dispenser mounted therein, coin tubes mounted in the main body and disposed at the bottom side of the coin dispenser, and sensing device including multiple optical sensor modules respectively aimed at the coin tubes. The distance between each optical sensor module and the coins in the respective coin tube is calculated by: measuring the time taken for the reflected light to travel from the coins in the respective coin tube to the proximity sensor of the respective optical sensor module and then multiplying the time thus measured by the speed of light. The number of coins in each coin tube is calculated by: deducting the distance between the respective optical sensor module and the coins in the respective coin tube from the pre-measured depth of the empty coin tube, and then dividing the reminder thus obtained by the thickness of single coin.