When you are done traveling and have reached your final destination, exit the turnstiles and then look for these machines. The machines can be found easily at almost all metro stations.
Next, simply insert your card into the slot which will be recycled and reused in the future for other travelers. Before you know it, a 500 won coin will drop down.
Your transportation card will not be returned to you. Don’t forget to pick up the coin that has dropped into the tray.