An opening/closing mechanism comprising: an intake member that accommodates a filter part and has an opening leading to a carburetor at an end wall portion facing the filter part; an opening/closing member that is disposed between the filter part and the end wall portion, and opens and closes the opening; and a working member that is disposed on a side opposite to the opening/closing member with the end wall portion interposed therebetween, the working member including an arm portion that extends along the end wall portion and is coupled with the opening/closing member at one end by sandwiching the end wall portion therebetween, and a holding portion that is provided at the other end of the arm portion, the intake member having a guide hole into which the arm portion is inserted to guide movement of the arm portion, characterized in that the guide hole has a first fixing portion that fixes the arm portion on one side in a movement direction of the arm portion, and a second fixing portion that fixes the arm portion on the other side in the movement direction of the arm portion, widths of the first fixing portion and the second fixing portion of the guide hole along a direction perpendicular to the movement direction of the arm portion are respectively equal to or less than a width of the arm portion along the direction perpendicular to the movement direction of the arm portion, and the guide hole includes a wide portion having a length equal to or greater than a length of the arm portion along a length direction, which is the movement direction of the arm portion, of the guide hole between the first fixing portion and the second fixing portion, and having a width greater than the width of the arm portion along the direction perpendicular to the movement direction of the arm portion. an intake member that accommodates a filter part and has an opening leading to a carburetor at an end wall portion facing the filter part; an opening/closing member that is disposed between the filter part and the end wall portion, and opens and closes the opening; and a working member that is disposed on a side opposite to the opening/closing member with the end wall portion interposed therebetween, the working member including an arm portion that extends along the end wall portion and is coupled with the opening/closing member at one end by sandwiching the end wall portion therebetween, and a holding portion that is provided at the other end of the arm portion, the intake member having a guide hole into which the arm portion is inserted to guide movement of the arm portion, characterized in that the guide hole has a first fixing portion that fixes the arm portion on one side in a movement direction of the arm portion, and a second fixing portion that fixes the arm portion on the other side in the movement direction of the arm portion, widths of the first fixing portion and the second fixing portion of the guide hole along a direction perpendicular to the movement direction of the arm portion are respectively equal to or less than a width of the arm portion along the direction perpendicular to the movement direction of the arm portion, and the guide hole includes a wide portion having a length equal to or greater than a length of the arm portion along a length direction, which is the movement direction of the arm portion, of the guide hole between the first fixing portion and the second fixing portion, and having a width greater than the width of the arm portion along the direction perpendicular to the movement direction of the arm portion.