A video encoding/decoding system includes a video encoding device, and a video decoding device. The video encoding device includes an encoding circuit for encoding an image including a diagnostic image or a normal image. The video decoding device includes a decoding circuit for decoding the image encoded in the encoding circuit, a check signal generation circuit for generating a check signal of the decoded image, a storage circuit for storing the check signal generated by the check signal generation circuit, and a comparison circuit for comparing the check signal stored in the storage circuit with the check signal generated by the check signal generation circuit. The failure is detected by comparing the check signal including an expected value stored in the storage circuit with the check signal including a comparison value generated by the check signal generation circuit by processing the same image data a plurality of times.