An operation guarantee system includes a decoder circuit, a comparison circuit, a CPU circuit, a frequency adjustment circuit and a DQ adjustment circuit. The comparison circuit compares a test data signal input from the decoder circuit with an expected value data signal input from the exterior, and detects the presence or absence of an output error in the decoder circuit. The CPU circuit controls the frequency adjustment circuit and the DQ adjustment circuit to vary a frequency of a clock signal input to an external memory and a delay amount of the data signal. In addition, the CPU circuit acquires a result of detection of the comparison circuit under various conditions. Then, the CPU circuit determines an appropriate frequency of the clock signal input to the external memory based on a relationship between the various conditions and the presence or absence of the output error.