A measuring device to measure the insulation resistance (Riso) of a fuel cell system with respect to an electrical ground point comprises a resistor arrangement--formed by a series connection of reference resistors--between two load-current lines of the fuel cell system as well as one reference voltage source, which is arranged between the ground point and a node between the reference resistors and which is switchable between at least two different reference voltages (Urefo, Urefu). The measuring device further comprises one voltage-sensing device that acquires the voltage (Uo, Uu) between at least one of the load-current lines and the ground point. Connected to the voltage-sensing device is an evaluation unit, which determines the insulation resistance (Riso) on the basis of a first voltage value, which represents a value of the voltage between a first of the load-current lines and the ground point while a first reference voltage (Urefo) of the reference voltage source is applied, and on the basis of a second voltage value, which represents the value of the voltage between the first load-current line and the ground point while a second reference voltage (Urefu) of the reference voltage source is applied.