Photoresponse was investigated for an amorphous oxide semiconductor, In–Ga–Zn–O, by the steady-state photoconductivity (SSPC) method. All the films exhibited extremely slow reversible photoresponses. Analysis of the transient photocurrent at varied temperatures provided similar activation energies of ~0.5eV for both the time constants and the photoconductivity. Mobility–lifetime (μτ) products were estimated from the photoconductivity spectra measured at the sweep rate of 2nm/s, which monotonically increased with increasing dark conductivity σ D (i.e. the Fermi level E F becomes shallower). The obtained μτ values are larger than those of hydrogenated amorphous silicon even if the E F dependence is considered.