A first connector housing is provided with a flexible lock arm. A tubular fitting detector is fitted to an outer periphery of the first connector housing while being slidable thereon between a first position and a second position. A retainer is provided on the fitting detector, and adapted to be engaged with a first portion of the lock arm in a case where the fitting detector is placed at the first position, and engaged with a second portion of the lock arm in a case where the fitting detector is placed at the second position. A second connector housing is provided with a projection adapted to release the engagement between the retainer and the first portion of the lock arm in a case where the second connector housing is completely fitted with the first connector housing, so that the fitting detector is allowed to slide from the first position to the second position. A finger pad is provided on the fitting detector, and arranged such that an operator's finger is placeable thereon while being separated from at least the lock arm, during a fitting operation between the first connector housing and the second connector housing.