A coupling connector includes: a connector housing of a first connector having an engagement piece formed by projecting an outer wall thereof; and a connector housing of a second connector having an engagement groove formed on an outer wall thereof. In the construction, the engagement piece is engaged with the engagement groove. The engagement piece and engagement groove are formed so as to be engaged with each other in an engagement direction intersecting perpendicularly to a coupling direction where the first and second connectors are coupled to each other. The connector housing of the first connector is slid to the coupling direction with respect to the connector housing of the second connector so that the engagement piece is engaged with the engagement groove so that the first connector and second connector are coupled to each other.