The optimal vessel pressure of the bioreactor for cell growth and the production of cellulase, as well as the effect of a shift in pressure within the reactor on cellulase production were investigated. The optimal vessel pressure for the cell growth of E. coli JM109/LBH-10 was 0.08 MPa, whereas that for the production of cellulase was 0.04 MPa. The maximal production of cellulase by E. coli JM109/LBH-10 with a shift in the vessel pressure from 0.08 to 0.04 MPa after 24 h was 636.8 U/mL, which was 1.2 times higher than that without a shift. The shift in vessel pressure optimized for cell growth to that for the production of cellulase after the mid-term log-phase resulted in higher cell growth and cellulase production. A simple process with a shift in the vessel pressure of bioreactors to enhance the production of cellulase from agricultural by-products has been developed and can be directly applied to the industrial-scale production of cellulases.