The pressure would increase. This can be explained by the ideal gas law where for an ideal gas its pressure, volume and temperature is a function of each other ans is expressed by an equation: PV = nRT. We first need to assume that this gas given is ideal. When the temperature and the number of moles is set to a constant value while the volume decreases, we can see that from the equation pressure and volume are indirectly proportional to each other, therefore as the volume decreases, the pressure inside increases.