A single-pass beam position monitor (SPBPM) was developed for measuring of the injection beam orbit of the KEK B-factory (KEKB), and 21 sets have already been installed. We adopted log-ratio processing using a logarithmic (log) amplifier for the SPBPM of KEKB. The measurement circuit consists of four log-amplifiers, four peak-hold circuits, four analog-digital converters (ADC), a memory circuit and a timing control circuit. Moreover, it coexists with the existing beam position monitor (BPM) system for a closed-orbit distortion (COD) measurement. In order to choose the BPM signal of the low energy ring (LER) and the high energy ring (HER) of KEKB, an RF signal switch circuit was also manufactured. These circuits were controlled by the same VXI standard as the existing BPM system. This report summarizes the SPBPM system and its performance.