with the hook projection 92 extending downward the end portions of the pull ropes 400 that extend out of the elongated sleeve 80 may be wound around both of the hook projections 92 and hooked on one of the hook projections 92 after the operating device 100 has been installed on a venetian blind the pull ropes 400 of the venetian blind pass through the tubular connector 301 the annular connector 30 the positioning tube 40 and the elongated sleeve 80 and have end portions hooked at the hook projection 92 of one of the retaining ring 90 as shown in fig5 normally the biasing arms 62 of the biasing spring 60 bias the movable roller 64 to move along the curved contact face 471 of the retaining protrusion 47 toward the stationary roller 50 and to be retained fittingly between the retaining protrusion 47 and the stationary roller 50 for clamping the pull ropes 400 between the movable roller 64 and the stationary roller 50 thereby positioning the slats under this condition the first and second insert pins 821 831 are respectively located at upper ends of the first and second radial slots 48 49 referring to fig7 when the slats are to be raised the end portions of the pull ropes 400 are released from.