A hydrogen leak detection system for a fuel cell vehicle system is provided to prevent burning or explosion outside the system causable by leaked hydrogen and to ensure safety of a fuel cell vehicle by detecting hydrogen leak. A hydrogen leak detection system for a fuel cell vehicle system includes: many hydrogen tanks(10) which store and supply hydrogen; a first valve(14) which is opened to control supply of the hydrogen tank when a key of a vehicle is on; a second valve(16) which is connected to the same line as the first valve to control supply of the hydrogen tank; a temperature sensor(12) which detects the temperature of the each hydrogen tank; a pressure sensor(20) which detects the pressures of the hydrogen tanks; a fuel cell stack(26) which reacts the hydrogen with oxygen to generate electricity; a fuel processing system(24) which supplies the hydrogen to the fuel cell stack; a low-pressure regulator(22) which regulates high-pressure hydrogen supplied from the hydrogen tanks to low pressure; and a controller which calculates the total amount of hydrogen supplied from the hydrogen tanks by subtracting the amount of hydrogen calculated from temperature and pressure values of the hydrogen tanks after given time, from the amount of initial hydrogen in the hydrogen tanks calculated from initial temperature and pressure values of the hydrogen tanks, and detects hydrogen leak to give an alarm by comparing a value, obtained by subtracting the total amount of hydrogen used in the fuel cell stack and the amount of hydrogen exhausted into the outside from the total amount of hydrogen supplied from the hydrogen tanks, with a permitted reference limit, wherein the total amount of hydrogen used in the fuel cell stack is obtained by accumulating electric currents generated in the fuel cell stack for the given time.