This page provides some brief summaries on traveling to IBS, Daejeon from Incheon International Airport (ICN). You can find more detailed descriptions in the link below. (If you come from Japan or China, Gimpo International Airport (GMP) can be more convenient.)
https://www.ibs.re.kr/eng/sub01_08_01.do
Option A. Direct Airport Limousine bus to Daejeon → IBS
1. (Terminal 1) Go to 1F exit No. 11 / (Terminal 2) Go to B1 bus stop No. 5
2. You can find a ticket box near the bus stop. Buy tickets for 'Daejeon'. If you use a ticket vending machine, select 'Chungcheong'→'Daejeon'.
3. Get off at Daejeon Government Complex(Daejeon-cheongsa)
4. Take a taxi and tell the driver you are heading to Institutes for Basic Science (IBS).
You can show the taxi driver below. (Korean address)
기초과학연구원으로 가주세요.
유성구 엑스포로 55 기초과학연구원
*First Bus - 6:20 (T1) 6:00 (T2) / Last Bus - 22:50 (T1) 22:30 (T2)
*Bus timetable: https://www.airport.kr/ap/en/tpt/busRouteList.do
(click 'Chungcheong' and go to 'Daejeon' tab)
*Bus Fare: 26,300 KRW
*Bus stop locations
Option B. Airport Limousine bus to Seoul Station → KTX Train to Daejeon → IBS
1-a. Take bus 6001 (17,000 KRW)
(Terminal 1) Go to 1F exit No. 5 / (Terminal 2) Go to B1 bus stop No. 29
1-b. Take bus 6702 (18,000 KRW)
(Terminal 1) Go to 1F exit No. 3 / (Terminal 2) Go to B1 bus stop No. 18
2. Get off at 'Seoul Station'.
3. Take KTX train to Daejeon. (~24,000 KRW)
Online ticket purchase: https://www.letskorail.com/ebizbf/EbizbfForeign_pr16100.do?gubun=1
4. Get off at 'Daejeon' station.
5. Take a taxi and tell the driver you are heading to Institutes for Basic Science (IBS).
Option C. AREX train to Seoul Station → KTX Train to Daejeon → IBS
1. Take AREX train (subway) to Seoul Station. You can choose Non-stop(express) or All Stop.
(Terminal 1) Across the street from Terminal 1 exits / (Terminal 2) Go to B1
2. Take KTX train tickets to Daejeon. (~24,000 KRW).
3. Get off at 'Daejeon' station.
4. Take a taxi and tell the driver you are heading to Institutes for Basic Science (IBS).
*subway station location
Terminal 1 1F 
Terminal 2 B1
