Disclosed herein is a method of generating an RTL description that implements any functional safety system. A high-level synthesis method for generating an RTL description in which a functional safety system is inserted by using an operation description defining a functional logic, a high-level synthesis script defining a high-level synthesis constraint, and a functional safety system implementation specification specifying a functional safety system to be inserted in a high-level synthesis process. The high-level synthesis method includes a control data flow graph generation step in which a high-level synthesis unit generates a control data flow graph using the operation description, and a first function safety system insertion processing step in which the high-level synthesis unit inserts the function safety system into the control data flow graph according to the function safety system implementation specification after the control data flow graph generation step.