In radiation therapy, there is no suitable method to confirm either the three-dimensional irradiated area or the absorbed dose during irradiation. If there were such a method it would allow verification of the absorbed dose as well as help to avoid extraneous irradiation. We previously proposed a method utilizing annihilation photons generated by therapeutic photon beams to measure the irradiated area directly during irradiation. In the present study, we designed and fabricated a fast YAP:Ce scintillation counter which uses a compact photosensor to measure annihilation photons. The energy resolution and detection efficiency were obtained experimentally using a (22)Na planar source. We measured count rates of background radiation by using the detector and estimated those of background radiation from an e-LINAC by correcting for detection efficiency. The obtained energy resolution of the YAP:Ce scintillation detector was 12.16+/-3.5% and its detection efficiency was 28.36+/-5.3%. The counting rates of background radiation from e-LINAC were approximately from 10(8) to 10(9) cps, and were proportional to the e-LINAC dose rate. Our developed detector had a high sensitivity, good time resolution and compact size which allows for easy radiation shielding. Therefore, we concluded that it was both practical and effective for the measurement of the position-distribution of annihilation photons.