A measuring system particularly suitably usable with an apparatus having a plurality of electron emitting sources, for measuring the characteristics of the electron emitting sources is disclosed. In the measuring system, the electron emitting sources are driven so that they emit electron flows of predetermined and different frequencies. The emitted electron flows are collected and, thereafter, only those signals having components corresponding to the predetermined frequencies are extracted. Based on the extracted signals, the characteristics of the electron emitting sources are measured at the same time and independently of each other.