One end portion of a connecting member 31 is connected to a first housing 1 such that the connecting member 31 is rotatable about a first rotation axis L1 fixed in position with respect to the first housing 1. The other end portion of the connecting member 31 is connected to a second housing 2 such that the connecting member 31 is rotatable about a second rotation axis L2 fixed in position with respect to the second housing 2 and extending parallel to the first rotation axis L1. The first housing 1 and the second housing 2 are located in different locations in a direction of the first and the second rotation axes L1, L2 such that a top surface 1a of the first housing 1 and an undersurface 2b of the second housing 2 are located generally on a same plane.