The present invention relates to a humidifying device for a breathing mask. The breathing mask includes a breathing tube assembled thereto. The humidifying device includes a body, a liquid discharger, a liquid receiver, and a water-absorbent member. The body is connected to the breathing tube. The body includes a passage communicating with the breathing tube. The liquid discharger is fixed to the body. The liquid discharger includes a liquid outlet communicating with the passage. The liquid receiver is fixed to the body and is disposed below the liquid discharger. The liquid receiver includes a liquid inlet communicating with the passage. The water-absorbent member consists of a filtering material. The water-absorbent member is accommodated in the passage and disposed between the liquid outlet and the liquid inlet.