A fuel cell system capable of improving performance and stability of the system by using stack off-gas includes: a power generation unit that generates power through an electrochemical reaction of a first fuel and a first oxidant; a reforming unit that supplies the first fuel to the power generation unit; a heating unit that receives second fuel and a second oxidant, combusts the second fuel, and is thermal-conductively coupled with the reforming unit; and a connection unit that connects the heating unit with the power generation unit to be in fluid communication and supplies off-gas of the power generation unit to the heating unit. The off-gas is supplied to the heating unit in a pulse type.