The South Korean intentional is in the UK to compete for his country at the Olympics.
The 22-year-old midfielder is expected to cost around £2m from Japanese side Cerezo Osaka.
"We're hopeful we'll be able to close that [deal] within the next few days," said Cardiff City chief executive Alan Whiteley.
"We think we've made good progress on the negotiations.
"Like all these things you always hesitate to say too much until the ink is on the paper.
"Obviously there's work permit issues as well but I think we've made pretty good progress this week."