The infrared absorbance of CaS (the B1 structure) in the wavenumber range is studied up to 17.0 GPa. The microscopic Grüneisen parameter at constant temperature T = 298 K, correlating the pressure dependence of the absorbance band with the pressure dependence of the unit cell volume, is in the range 1.42-1.62. There is no indication for pressure-induced opaqueness for the sample or loss of infrared intensity in the entire pressure range studied here.