A mass analyzer includes two rotating electric field (REF) units, sinusoidal signal generators and a means for separation of dispersed ions. The REF units include a plurality of elongated electrodes surrounding a central axis, and are lined in tandem at elongated direction. Sinusoidal signals are applied to the electrodes to rotate electric fields within each REF unit. The means for separation is placed adjacent the downstream end of the 2nd REF unit. Ions enter the 1st REF unit, diverge outwards and leave the 1st REF unit on off-axis positions. The ions successively enter the 2nd REF unit and converge inwards because of 180 degrees phase difference from the 1st REF unit. Specified mass ions return to and travel along the central axis. However, unspecified mass ions deviate from the central axis and travel apart from the central axis. The means for separation separates specified ions from unspecified ions.