An injector of an electron linear accelerator has been modified at ISIR of Osaka University in order to increase a single bunch charge from 14 nC to 60 nC. A 6th subharmonic prebuncher has been replaced with two 12th subharmonic prebunchers and a 6th subharmonic prebuncher which are newly constructed. A one-dimensional disk model has been used to calculate the bunching of the beam and to decide the optimum location of the subharmonic prebunchers. The subharmonic prebunchers are immersed in a solenoidal magnetic field so that the electron beam is confined during the travel through the drift region. The single bunch of 16 - 20 ps duration and up to 67 nC in charge, with the energy spread of 0.7 - 2.5 % over the range of 24 - 34 MeV, and a repetition rate from a single shot to 720 pps can be obtained. The energy spread depends on the charge and the minimum spread is 0.7 % at 33 nC. The single bunch of 25 - 45 nC in charge is used for the experiments in routine work.