There is provided is a radiation image acquiring device which corrects a positional displacement between a collimator and a detector and obtains an image without artifacts. The device includes a detector (21) to measure a radiation; a collimator (26) including a through-hole (27) having one or more detectors (21) disposed therein and configured to limit an incident direction of the radiation; a positional displacement measuring unit configured to measure a positional displacement between the detector (21) and the collimator (26) by use of a profile of a radiation source measured by the detector (21) based on the radiation source disposed corresponding to a predetermined detector (21).