To realize a coordinate reading apparatus which is capable of easily setting a particular function to a switch provided on a coordinate indicator, and an interface unit and a coordinate reading system for the coordinate reading apparatus. A coordinate detecting section 21 detects a coordinate value for a position indicated by a coordinate indicator 3 and a switch status. A function selecting means 22 determines that a menu provided on a tablet 2 is read from the coordinate value and the status, to store functional data assigned to the menu in a memory means 23. A conversion status determining means 24 determines that the inputted status coincide with the status to be converted stored in the memory means 23, to notify coincidence to a conversion status output means 25. Receiving the notification, the conversion status output means 25 outputs data to be outputted stored in the memory means 23 instead of inputted switch status.