This paper presents an approach to build synsets for Indonesian WordNet semi-automatically using monolingual lexical resources available freely in Bahasa Indonesia. Monolingual lexical resources refer to Kamus Besar Bahasa Indoensia or KBBI (monolingual dictionary of Bahasa Indonesia) and Tesaurus Bahasa Indonesia (Indonesian thesaurus). We assume that monolingual resources will play an important role in synsets building, because it will provide more accurate senses specifically for Bahasa. Besides, resources that have been used are produced by Bahasa Indonesia Language Center, which is a government institution that manages Bahasa Indonesia development. However, the synsets retrieved will be considered as a prototype version of Indonesian WordNet synsets.