This paper reports theoretical calculations of ray and wave aberrations of a focusing grating coupler (FGC) which is used as an objective in the integrated-optic disk pickup device proposed by the authors. Although a FGC is theoretically aberration free, characterizations of aberrations caused by fabrication errors are necessary to reproduce the device. Primary aberrations were discussed with ray aberrations, and astigmatism and coma were predicted to be essential and effective. The upper limits of allowable fabrication errors were estimated using the calculated results of wave aberrations. The calculation methods and results are basic and useful when designing various devices using the FGCs.