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 (A) 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 (B) 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 (C) equal to or greater than a length (D) 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 (E) greater than the width (B) 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 (A) 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 (B) 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 (C) equal to or greater than a length (D) 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 (E) greater than the width (B) of the arm portion along the direction perpendicular to the movement direction of the arm portion . <EOS>