PROBLEM TO BE SOLVED: To provide a radiation detector capable of reducing the occurrence of artifact, and a radiation image photographing device using the same.SOLUTION: An electronic cassette acting as a radiation image photographing device includes: a scintillator for converting a radioactive ray into visible light and a photodetection substrate that is provided with a photodiode for converting the visible light into an electric charge. For the scintillator and the photodiode, when setting a temperature change ratio of sensitivity to the radioactive ray of the scintillator as A[%/K] and setting a temperature change ratio of sensitivity to the visible light of the photodiode as B[%/K], such a combination that A and B satisfy an expression of -0.35[%/K]
