An electrical connector assembly comprising: a first connector including a first housing; and a second connector including a second housing and capable of fitting to the first connector in a fitting direction along an axial line thereof, wherein said first housing includes a slit groove and a flexible section formed with the slit groove, said flexible section including a locking section protruding inwardly in a radial direction of the first housing, said second housing includes a locked section, said second housing includes a guiding groove for guiding the locking section toward said locked section, during the connector fitting process, wherein said guiding groove has a groove width decreasing toward the locked section and has slanted side edges forming a generally V-shape when viewed in the radial direction, and said locked section includes an engaging section for engagement with the locking section formed on said flexible section of the first housing for elastically deforming the locking section in the radial direction when the first housing is moved in a fitting direction to the locking position and locks with the locking section in a connector-removal direction, wherein said engaging section is formed in a step shape as a concave section recessed in an outer surface of the second housing, wherein said concave section includes a slanted side surface for guiding and elastically deforming the locking section so that the locking section is released from the locked section in the radial direction when the first housing is rotated relative to the second housing, wherein the first housing is composed of a sleeve-like outer housing and a column-like inner housing provided in the outer housing, wherein the slit groove and the flexible section are formed on the outer housing, wherein the outer housing is attached on the inner housing allowing rotation of the outer housing relative to the inner housing around the axial line, wherein on an outer circumferential surface of the inner housing is provided an annular groove for connecting with the outer housing which is formed by a first annular protrusions and a second annular protrusions that are adjacent to each other in the direction of the axial line, wherein on the annular protrusion is formed a cut-out section (17A -1) having less protrusion in the radial direction at a plurality of points in the circumferential direction. a first connector including a first housing; and a second connector including a second housing and capable of fitting to the first connector in a fitting direction along an axial line thereof, wherein said first housing includes a slit groove and a flexible section formed with the slit groove, said flexible section including a locking section protruding inwardly in a radial direction of the first housing, said second housing includes a locked section, said second housing includes a guiding groove for guiding the locking section toward said locked section, during the connector fitting process, wherein said guiding groove has a groove width decreasing toward the locked section and has slanted side edges forming a generally V-shape when viewed in the radial direction, and said locked section includes an engaging section for engagement with the locking section formed on said flexible section of the first housing for elastically deforming the locking section in the radial direction when the first housing is moved in a fitting direction to the locking position and locks with the locking section in a connector-removal direction, wherein said engaging section is formed in a step shape as a concave section recessed in an outer surface of the second housing, wherein said concave section includes a slanted side surface for guiding and elastically deforming the locking section so that the locking section is released from the locked section in the radial direction when the first housing is rotated relative to the second housing, wherein the first housing is composed of a sleeve-like outer housing and a column-like inner housing provided in the outer housing, wherein the slit groove and the flexible section are formed on the outer housing, wherein the outer housing is attached on the inner housing allowing rotation of the outer housing relative to the inner housing around the axial line, wherein on an outer circumferential surface of the inner housing is provided an annular groove for connecting with the outer housing which is formed by a first annular protrusions and a second annular protrusions that are adjacent to each other in the direction of the axial line, wherein on the annular protrusion is formed a cut-out section (17A -1) having less protrusion in the radial direction at a plurality of points in the circumferential direction.