The dependence of the intersubband transitions on pressure in strained In0.15Ga0.85As/GaAs multiple quantum wells has been studied in two samples with well widths of 8 and 15 nm, respectively, with photomodulated transmission spectroscopy by using a diamond anvil cell. The pressure coefficients of the energies for the intersubband transitions were found to depend significantly on the well widths and to be smaller than that of the band gap of constituents in bulk form. These results suggested that the critical thickness for strained In0.15Ga0.85As/GaAs layer should be smaller than 15 nm.