To minimize friction of a cover part in transferring from an inclination stage to a slide stage or reverse transferring by feeding a fixing means for simply interlocking a transfer carriage and a cover carrier adjusting means to incline a cover part.

A fixing means 34 is guided with low friction in a horizontal slot of a transfer carriage 30. The fixing means 34 is one piece having the inclined surface corresponding to a capture limit strap on the transfer carriage 30 and a switching surface 36B on a guide element 32 at both ends thereof. By this arrangement, the roof opening part is gradually closed, so that when the transfer carriage 30 is slid forward from the open position, the fixing means 34 returns to the non-fixing position. In this process, the external area of a first cam 22 is moved to a third guide curve 40, so that resistance produced between the transfer carriage 30 and a cover transport element 20 is such a degree that the fixing means 34 sideslips to a recess 36A of a control means 36.