BANGKOK — The mother of a pro-democracy activist faces up to 15 years in prison after acknowledging that she had received a private message on Facebook that the police say insulted Thailand’s monarchy.

The activist’s mother, Patnaree Chankij, 40, who works as a maid, will be tried by a military court under Thailand’s lèse-majesté law, which makes it a crime to insult the long-reigning King Bhumibol Adulyadej, the queen or the crown prince.

On Saturday, human rights activists called Ms. Patnaree’s arrest a day earlier a “new low” for Thailand, which has increased prosecutions under the lèse-majesté law since the military took power in 2014.

Poonsuk Poonsukcharoen, a lawyer advising Ms. Patnaree, said Ms. Patnaree had sent only a one-word reply, “Ja,” acknowledging receipt of the Facebook message, similar to saying “Yeah,” but had not expressed agreement with it or commented on its content. The message was not made public, so as not to repeat the alleged insult, as is typical in such cases. The sender of the message, Burin Intin, 28, was arrested last month.