A pressure-distribution analysis was carried out for a vacuum unit of the KEK 2.5-GeV linac in order to improve the pressures at the radio frequency (rf) windows and accelerator columns. The characteristic matrix ([G]) of the vacuum network geometry including the pumps was calculated for several configurations of the pumps. The results show that three additional pumps must be installed near to the rf window, so as to suppress any pressure rise due to outgassing in the long waveguides. A distributed configuration of the pumps causes more substantial reduction of the pressures at every node. However, it cannot cause a drastic decrease in the pressure at the accelerator-column center, because of its poor conductance. Pressure measurements at the nodes have also been carried out for these configurations.