A display device includes a display panel and a timing controller. An optimum combustion air-fuel ratio is then estimated based on the detected hydrogen concentration. Each step has a first folding end and a second folding end. The plate can be held in an initial position wherein the first and second windows are aligned, whilst the third and fourth windows are not aligned. The gradients of road surfaces might be judged without using various sensors but only on the basis of information which can be obtained from a speed detector for detecting wheel speeds.