A cam mechanism is provided in a DUT board mounting portion of a test head which includes a cam shaft having a guide pin connected to an actuator and a driven ring having a sloped groove along which the guide pin is guided, and a second guide pin provided on an outer cylindrical surface of the driven ring which is guided along a substantially L-shaped cam groove formed in a cylindrical opening of the DUT board mounting portion of the test head. When the second guide pin is located on a horizontal portion of the L-shaped cam groove, the driven ring is inhibited from moving downwardly but is rotated together with the cam shaft so that an engaging member of the driven ring is caused to engage with the DUT board. When the second guide pin reaches a vertical portion, only the cam shaft is rotated, and the driven ring moves downwardly along the vertical portion of the L-shaped groove to urge the DUT board assembly against contact probes on the test head assembly.