Hi @JitHoon ,
Thanks for your post and welcome to the Cesium community.
One approach used in the past to accomplish this is using sampleTerrainMostDetailed. It is discussed here in this previous forums post elevation graph on cesium earth - #2 by ashwini_biradar and demonstrated in this sandcastle example Cesium Sandcastle.
Please let us know if those examples are useful or if you have further questions.
Thank,
Luke