A vehicle body top structure for enhancing the rigidity and strength of the intersections of side rails and a front rail. The vehicle body top structure is provided with left and right side rails provided so as to extend in the front-and-rear direction at the top part of a vehicle body side part, and a front rail extending left and right, one end part of which is joined to the left side rail, and the other end of which is joined to the right side rail. Thin-plate parts having a smaller plate thickness than thick-plate parts and thick-plate parts are disclosed at the one end and the other end of the front rail. The thin-plate parts are each connected to the left and right side rails.