An operation method at the time of load increase of fuel cell system includes in this order a first step of determining a target power generation amount of the fuel cell module, a second step of increasing the flow rate of the oxygen-containing gas supplied to the fuel cell module, a third step of increasing the flow rate of the water supplied to the fuel cell module, a fourth step of increasing the flow rate of the fuel gas supplied to the fuel cell module, a fifth step of increasing the power generation amount of the fuel cell module, and a sixth step of detecting whether the power generation amount of the fuel cell module reaches the target power generation amount or more.