To measure X-ray spectra with high count rates, we developed a detector consisting of a Lu2(SiO4)O LSO crystal with a decay time of 40ns and a multipixel photon counter (MPPC). The photocurrents flowing through the MPPC are converted into voltages and amplified by a high-speed current–voltage amplifier, and event pulses from the amplifier are sent to a multichannel analyzer to measure spectra. We used three MPPCs of 100, 400 and 1600pixels/mm2, and the MPPCs were driven under pre-Geiger mode at a temperature of 20°C. At a tube voltage of 100kV and a tube current of 5.0μA, the maximum count rate was 12.8kilo-counts per second. The event-pulse widths were 200ns, and the energy resolution was 53% at 59.5keV using a 100-pixel MPPC. •Zero-dark-counting X-ray detection was performed under pre-Geiger mode.•200-ns-width event pulses were produced by a high-speed amplifier.•X-ray spectra were measured using three LSO-MPPC detectors.•Energy resolution improved with decreasing MPPC pixel number.