The dependence of the intersubband transitions on pressure in strained In_(0.15) Ga_(0.85)As/GaAs multiple quantum wells has been studied in two samples with well widths of 8nm and 15nm, respectively, with photo- modulated transmission spectros copy by using a diamond anvil cell. The pressure coefficients of the energies for the intersubband transitions have been found to depend significantly on the well widths and be smaller than that of the band gap of constituents in bulk form. These results suggest that the critical thickness for strained In_(0.15)Ga_(0.85)As/GaAs layer should be smaller than 15nm.