Abstract: A humidity modifying auxiliary tool having a superior humidity modifying effect is provided. A humidity modifying auxiliary tool capable of being installed in a condition where the humidity modifying auxiliary tool covers a sound hole provided on a body part of a stringed instrument includes: a lid portion capable of covering the sound hole; a frame portion provided at a back side of the lid portion, the frame portion being equipped with a humidity modifying sheet retaining portion to retain a humidity modifying sheet to modify humidity of the body part.