He pitched in the KBO League for the Doosan Bears, Kia Tigers, and SK Wyverns Drafted by the Doosan Bears with the 1st pick in the second round of the 2003 KBO Draft, Jun made his pro debut on September 2, 2003.

In 2005, Jun was traded to the Kia Tigers for Daniel Rios, and had his statistically outstanding season as a setup man and closer, going 3-2 with 5 saves and a 3.00 ERA in 57 innings pitched.

At the 2006 WBC, he played an important role as a left-handed specialist, appearing in the last three games for the South Korean team.

Sidelined by injuries, he only appeared in 10 games during the season, going 2-4 with a 4.64 ERA and 29 strikeouts in 33 innings pitched.

He was runner-up in ERA (3.11) behind Wyverns ace Kim Kwang-Hyun and fifth in strikeouts (136), going 8-4 with 8 saves.