The generation mechanism of the screech tone in the helical oscillation mode is investigated using a series of instantaneous schlieren photographs. From the photographs, six evanescent sound sources are observed as prominent points along the jet axis. These sound sources move along circular orbits in the planes perpendicular to the jet axis just downstream of the rear edges of each shock cell. The speed of moving sound sources is supersonic and the moving Mach cones generated behind the moving sound sources form the helical-shaped wave front of the screech tone. Existence of the moving Mach cones about the jet axis was confirmed by comparing schlieren photographs and drawings of the envelopes of the moving Mach cones. The helical vortical structure appears to overlap on the envelopes of the moving Mach cones.