A program optimization system is provided with a CNC simulator configured to sequentially read out a machining program and perform a machining simulation, a machining program storage unit configured to successively transfer the machining program to the CNC simulator, and a transfer speed control unit configured to control a transfer speed to be a predetermined lower limit value. The CNC simulator optimize the machining program when a state of buffering deficiency in which the machining program to be read out is insufficient is detected in a cutting section and create optimized machining program free from a buffering deficiency.