The hard X-ray detector (HXD-II) is one of the scientific payloads onboard Suzaku, the 5th Japanese cosmic X-ray satellite. After the launch in July 2005, all the HXD-II components, including the sensors and analog/digital electronics, have been working normally. In order to archive the maximum performance of the HXD-II, especially the GSO/BGO well-type phoswich counters, extensive in-orbit qualification and calibration have been carried out utilizing the data acquired in early operations. Major items of these efforts include; to estimate the circuit dead time, calibrate energy scale, optimize the event selection criteria for background reduction, study the background, and examine the detector response. As a result of these in-orbit calibrations, the HXD-II background in the 10 ∼ 600 keV range has been successfully lowered to (0.5 - 5.0) × 10-4 c s-1keV-1cm-2. This is the lowest among the background ever achieved in orbit by cosmic hard X-ray detectors.