At the KEKB accelerator, electrons and positrons are injected from the LINAC to the storage ring through the beam transport (BT) line. LINAC had continuously injected each beam alternately every about a few minutes. In our linacs system, it is very important to switch acceleration devicesfrom electron mode to position and vica versa to keep high storage currents. To attain this, we have developed new readout system for the beam position monitors (BPMs) at the BT line, by which the fast monitoring is possible. For the old BPM system, it had been taken a few seconds to switch the beam modes and about one second to measure the beam positions. With the new system, measuring interval was achieved to be 20 msec, including switching time, and we demonstrated that new system contributed to stable beam operation. In this manuscript, we will report on a detail of the fast readout for the BT BPM system including its performance.