The aim of this study was to produce galactooligosaccharides (GOS) from lactose using ${eta}$-galactosidase from Aspergillus oryzae immobilized on a low-pressure plasma-modified cellulose acetate (CA) membrane. Specifically, a novel method was developed for multilayer enzyme immobilization involving polyethyleneimine (PEI)-enzyme aggregate formation and growth on a CA membrane. A large amount of enzyme (997 ${mu}g/cm^2$ membrane) was immobilized with 66% efficiency. The $K_m$ value for the immobilized enzyme was estimated to be 48 mM, which indicates decreased affinity for the substrate, whereas the Vmax value was smaller. The immobilized enzyme showed good storage and operational stability. The half-life of the immobilized enzyme on the membrane was about 1 month at $30^{circ}C$ and ~ 60 h at $60^{circ}C$. Maximum GOS production of 27% (w/w) was achieved with 70% lactose conversion from 320 g/L of lactose at pH 4.5 and $60^{circ}C$. Trisaccharides were the major types of GOS formed and accounted for about 75% of the total GOS produced. Based on these results, immobilized enzyme technology could be applied to GOS production from lactose.