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Brilliant synchrotron radiation (SR) has enabled us to investigate fast phenomena even in small samples. At the same instance, the property of sample may be affected by the SR due to the high brilliance. An SR X-ray pump and optical probe measurement has then been performed at a storage ring facility of SPring-8 to investigate the spectral change in the near infrared region of a GaAs single crystal irradiated by an intense X-ray pulsed SR beam with a photon energy of just above the K-edge of As. The transient transmission spectrum was monitored by using a broadband optical pulsed beam and a dispersive spectrometer. We obtained the spectral profile change with a response time of shorter than the pulse duration of SR. The spectrum shows the abrupt change in transmittance at around 880 nm corresponding to the band gap energy of GaAs, and the fringe on the spectrum originated from the interference at the sample with a thickness of about 10μm. The band gap energy and the fringe shift to the opposite direction to each other, the behavior of which is explained to be due to the band gap shrinkage accompanied with the decrease of refraction index originated from the free carriers. The transient band gap shrinkage suggests the nonthermal lattice distortion induced by SR irradiation. [ABSTRACT FROM AUTHOR]