BEIJING — A Tibetan entrepreneur who is a vocal but moderate advocate for bilingual education in schools across Chinese-ruled Tibetan regions has been illegally detained by the police for one and a half months, his family said.

The man, Tashi Wangchuk, 30, who lives with his parents in the western town of Yushu, has written about language policy on his microblog. He has highlighted the dearth of meaningful Tibetan language education and expressed concern that many Tibetan children are unable to become fluent in their native language, a widespread worry in the ethnic group.

Mr. Tashi was detained on Jan. 27 and has been held for 44 days. According to Chinese law, the police can generally detain a person for 30 days before officers must ask prosecutors to bring formal arrest charges or release the person. Prosecutors then have seven days to announce a charge.

Mr. Tashi’s family has tried contacting the Yushu police and the town’s main detention center, where relatives say they believe he is, but officers have not given them a reason for the detention and have not let them see Mr. Tashi.