A mounting unit for an electromagnetic contactor that can prevent erroneous mounting of a non-mountable unit in a state in which a mountable unit is mounted on a case-side mounting portion, and a connection structure of an electromagnetic contactor that uses the mounting unit. A mounting unit (3) for an electromagnetic contactor can be mounted on electromagnetic contactors (2a, 2b) having on a unit mounting portion (6b) a contactor-side mounting preventing portion (6f) that prevents mounting by engagement when a non-mountable unit, which cannot be mounted, is mounted. In the mounting unit (3), a unit main body (39) is provided with a mounting preventing transfer member (51) that forms a unit-side mounting preventing portion (54) that engages with a contactor-side mounting preventing portion (6f) when mounted on the electromagnetic contactors (2a, 2b) and prevents mounting of the non-mountable unit on a unit mounting portion (3g) onto which the non-mountable units (5a, 5b) are mounted.