Apostolov joined Beroe Stara Zagora|Beroe] when he was eight years old and signed his first contract in 2006. He made his A PFG debut on 10October 2007 in a 1–0 away defeat against CSKA Sofia. Apostolov would play in 18 more league matches during the season, scoring two goals. In June 2009 he was transferred to OFC [Sliven 2000], but six months later returned to Beroe. In the end of the 2009–10 season he and his team won the Bulgarian Cup.