An electron beam (EB) prober for VLSI internal diagnosis has been developed. The essential requirements for an EB prober are quantitative voltage measurement, waveform observation with high time resolution, and a diagnostic function for locating faults easily. The newly developed EB prober satisfies these requirements. It has an improved energy analyzer for quantitative voltage measurement, a stroboscopic function with 1 ns time resolution, and a unique diagnostic function wherein voltage images of LSIs are compared. This voltage image comparison provides easy detection of faulty areas by comparing voltage images of both faulty and normal LSIs. In this respect this EB prober is the first machine which considers locating faults easily in a large VLSI chip.