A startup control device of a fuel cell system includes a fuel cell stack having a plurality of fuel cells stacked together, each of the fuel cells having a membrane electrode assembly including an electrolyte membrane interposed between a cathode electrode and an anode electrode. The device further includes a gas supply unit configured to supply a gas to the fuel cell stack and a controller configured to control the gas supply unit based on a parameter value related to the occurrence of flooding of the fuel cell stack. During startup of the fuel cell stack, the gas is supplied to the fuel cell stack at a first supply pressure for a time period determined based on the parameter value, and after the time period has expired, gas is supplied to the fuel cell stack at a second supply pressure, the second supply pressure being less than the first supply pressure.