This paper explores Korean substitute `keosigi` which changes to discourse markers. In order to understand the changes of meanings and functions of substitute `keosigi`, I introduced the approach of Grammaticalization which is defined as "an increase of the range of a morpheme advancing from less grammatical to a more grammatical status (Kurylowicz 1965:52)". `keosigi` which is grammaticalized to discourse marker has originated from noun-substitute. Substitute `keosigi` can take any particle any postposition. It means [deixis (very weak)], [definiteness] and [substitution]. When substitute `keosigi` is grammaticalized to discourse marker, it changes like follows; Firstly, `Keosigi` can`t take any of case particle and postposition. Secondly, Its original meanings- [deixis], [definiteness] and [substitution]- are bleached. Thirdly, It gains new meanings; [gaining time] and [hesitation].