In an SI engine, increasing the compression ratio could be one mean of achieving higher thermal efficiency. However, when the compression ratio is increased, knock occurs and it prevents higher thermal efficiency. It is generally known that if the burning velocity is increased and the combustion period is shortened, the occurrence of knock may be suppressed. Here, hydrogen was added to the gasoline engine as a mean of increasing the burning velocity. As a result, it has been confirmed that the occurrence of knock could be controlled to some extent, and knock could be completely avoided depending on the conditions for the distribution of hydrogen. Furthermore, it became clear that this result might have originated not only by the increase in the burning velocity but also by the hindrance of radical production by the hydrogen.