A fuel cell system comprising: a fuel cell supplied with reformed gas and oxidant gas, and generating electric power by an electrochemical reaction; a reforming portion configured to generate the reformed gas from a fuel and reforming water, and to supply the generated reformed gas to the fuel cell; a water reservoir configured to accommodate the reforming water and to allow the reforming water to overflow from an overflow position (Lo) in a case where the reforming water exceeds a capacity of the water reservoir; a condenser configured to supply condensed water to the water reservoir, the condensed water being formed by condensation of steam included in the reformed gas flowing through the reforming portion and the fuel cell; a water supply pipe configured to be connected to a high pressure water supply source (Sw) via a water purifier and to supply tap water to the water reservoir; a water supply valve provided at the water supply pipe, and opening and closing the water supply pipe; a pump (11b2) including an inlet port and an outlet port, the inlet port being arranged at a bottom portion of the water reservoir and the outlet port being in fluid communication with the reforming portion; a low position detection sensor configured to detect an abnormal level, pumping up of the reforming water stored in the water reservoir to the reforming portion by the pump (11b2) being difficult in a case where a water level of the reforming water stored in the water reservoir is at the abnormal level, since the inlet port is arranged at the bottom portion of the water reservoir; a high position detection sensor configured to detect a normal level, the water level of the reforming water stored in the water reservoir exceeding a high position between the overflow position (Lo) and the abnormal level at the normal level; and when the fuel cell system in a power generation state in which the electric power is generated by the steady operation, a control apparatus configured to determine that the water level is in a normal range and to control an output of the fuel cell to be in a steady state, in a case where the high position detection sensor detects the normal level, the control apparatus configured to determine that the water level is in an output limited range and to reduce the output of the fuel cell, in a case where the low position detection sensor does not detect the abnormal level and the high position detection sensor does not detect the normal level, wherein the fuel cell system is configured, in a case where the fuel cell system is newly installed or the fuel cell system that has been used is moved and is installed at a new location, to start the operation of the fuel cell system, the tap water, of which the ionic component has been removed, is supplied from the water supply pipe to the water reservoir, characterized in that the control apparatus is configured, from immediately after the high position detection sensor has detected the normal level, to make the water supply valve be in an open state for a predetermined time period (ta) and to supply the water of a first predetermined amount to the water reservoir, and an amount of the water supplied to the water reservoir in the normal range corresponds to water of the first predetermined amount, the first predetermined amount corresponds to the amount of water needed to re-start the fuel cell even after the fuel cell is started and stopped one time. a fuel cell supplied with reformed gas and oxidant gas, and generating electric power by an electrochemical reaction; a reforming portion configured to generate the reformed gas from a fuel and reforming water, and to supply the generated reformed gas to the fuel cell; a water reservoir configured to accommodate the reforming water and to allow the reforming water to overflow from an overflow position (Lo) in a case where the reforming water exceeds a capacity of the water reservoir; a condenser configured to supply condensed water to the water reservoir, the condensed water being formed by condensation of steam included in the reformed gas flowing through the reforming portion and the fuel cell; a water supply pipe configured to be connected to a high pressure water supply source (Sw) via a water purifier and to supply tap water to the water reservoir; a water supply valve provided at the water supply pipe, and opening and closing the water supply pipe; a pump (11b2) including an inlet port and an outlet port, the inlet port being arranged at a bottom portion of the water reservoir and the outlet port being in fluid communication with the reforming portion; a low position detection sensor configured to detect an abnormal level, pumping up of the reforming water stored in the water reservoir to the reforming portion by the pump (11b2) being difficult in a case where a water level of the reforming water stored in the water reservoir is at the abnormal level, since the inlet port is arranged at the bottom portion of the water reservoir; a high position detection sensor configured to detect a normal level, the water level of the reforming water stored in the water reservoir exceeding a high position between the overflow position (Lo) and the abnormal level at the normal level; and when the fuel cell system in a power generation state in which the electric power is generated by the steady operation,