Provided is a wet state control method for a fuel cell system in which cathode gas is supplied to a fuel cell while the cathode gas partially bypasses the fuel cell, the wet state control method being for controlling a wet state of the fuel cell by adjusting wet control parameters including a bypass valve opening degree, a cathode gas pressure, and a cathode gas flow rate. When the fuel cell is controlled to a wet side, at least either one of the cathode gas flow rate and the cathode gas pressure is adjusted in priority to adjustment of the bypass valve opening degree.