A board-to-board connector comprising a first connector and a second connector is disclosed. The first connector has a first housing mounted on first terminals. The second connector is configured to be engaged, by fitting, with the first connector. The second connector has a second housing mounted on second terminals, which are configured to make contact with the first terminals. At least a part of an entrance end portion a concave guide portion is provided with a sloped guide surface formed therein. Due to the insertion of a second fitting guide portion into a concave guide portion, in such a manner that portions of the second fitting guide portion are brought into abutting contact with the sloped guide surface, the first and second terminals are spaced apart from each other in a state where fitting faces of the first and second housings are even with each other.