A real-time monitoring of an immersed antenna type inductively coupled plasma (ICP) was done with optical emission spectroscopy (OES) to check the reports that sputtered atom density is decreasing as the ICP power is increased. At 10 mTorr pressure of Ar, Mg was sputtered by a bipolar pulsed power supply into 2 MHz ICP which has an insulator covered 2.5 turn antenna. Emitted light was collected in two different positions: above the target and inside the ICP region. With 100 W of Mg sputtering power, the intensities of Mg I (285.06 nm), Mg II (279.48 nm), Ar I (420.1 nm) were increased constantly with ICP power from 100 W to 600 W. At 500 W, the intensity of $Mg^+$ exceeded that of Mg under PID controlled discharge voltage of 180 V. The ratio of Mg II/Mg I was increased from 0.45 to 2.71 approximately 6 times.