Temporary placement structure for sub cover

A pair of opposing inside portions in an opening (21) of a main cover are provided with groove forming portions which define engagement grooves (49A, 49B), respectively. Engagement shafts (50A, 50B) which can be engageably inserted into the engagement grooves (49A, 49B) from the outside and have a thickness larger than the width of the engagement grooves (49A, 49B) are provided on side portions of a sub cover (40) opposing the inside portions. The groove forming portions have elasticity. When the engagement shafts (50A, 50B) are engageably inserted from the outside, the groove forming portions deform so as to allow engageable insertion of the engagement shafts (50A, 50B). After the engagement shafts (50A, 50B) have been engageably inserted, the groove forming portions are restored due to the elasticity thereof, and lock the engagement shafts (50A, 50B) to prevent separation of the engagement shafts (50A, 50B) in the opposite direction, and support the sub cover (40) so as to be openable with the engagement shafts (50A, 50B) being locked.