error invalid multibyte string using tm packages in R

Question

I am studying R as a beginner, try to make wordcloud. Using tm and wordcloud packages. I did change encoding to UFT-8,try to many solutions(remove encoding, using Sys.setlocal code, stringr...) to solve the error.

Here is my code

library(tm)
library(SnowballC)
library(RColorBrewer)
library(wordcloud)

filePath<-'C:/Users/aaa/Desktop/강의희망.txt'
text <- readLines(filePath)
docs <- Corpus(VectorSource(text))

#the error is occured here

docs <-tm_map(docs,content_transformer(tolower)) 

The text file is maked up with unsorted Korean words. What should I do?