When an electric connector assembly is in a connector fitting state, a distance from a receptacle protruding wall first end surface on one end side of a receptacle housing in a longitudinal direction to an inner surface of a receptacle end wall on the other end side and a distance from a plug end wall first inner surface on one end side of a plug housing in the longitudinal direction to an outer surface of a plug end wall on the other end side are equal to each other at an optional position in an upper-lower direction.