To prevent abnormal combustion such as after-burning by providing a check valve provided on an intake side or an exhaust side capable of obtaining favorable air having no ventilation resistance and low fuel economy on the intake side by controlling an intake or exhaust opening part closely by a variable valve and ensuring favorable exhaust efficiency by cadenacy effect on the exhaust side to prevent back flow between intake air and exhaust air.

In this valve device having at least one opening part 17A, 17B, 17C, 17D provided in an intake port 3a or an exhaust port 3b, the variable valve 15 capable of opening and closing at least one opening part 17A, 17B, 17C, 17D steplessly in accordance with fluctuation of an engine 1 is provided.