A keyboard system for an electronic musical instrument includes: a keyboard having a number of keys; a keyboard central processing unit (CPU) coupled to the keyboard; a switch connected to each key on the keyboard; a pull-up resistor serially connected to each switch; a unit for applying voltage to a serial circuit of each switch and each pull-up resistor; and a unit for parallel supplying a signal at an intersection point between each switch and each pull-up resistor to the CPU. The operation states of a plurality of keys can be independently detected by sequentially detecting the potential at the interconnection points.