[Problem] To provide: an antimicrobial agent which utilizes an antimicrobial activity and a pharmacological activity of a plant belonging to the family Juncaceae, comprises a material produced by seeding a seed culture of mushroom mycelia to a plant belonging to the family Juncaceae, culturing the mycelium while adding a nutrient source to thereby proliferate the mycelium and then drying the proliferated mycelium, can exhibit an excellent antimicrobial activity even when used in a small amount, can be added to a food, can be sprayed onto an agricultural crop or soil, and can be used as a therapeutic agent for athlete's foot； and a method for producing the antimicrobial agent. [Solution] A plant belonging to the family Juncaceae which is cut into a predetermined length or which is cut into a predetermined length and then milled is mixed with a nutrient source for a seed culture of mushroom mycelia, and the seed culture of mycelia is inoculated to the mixture of the plant belonging to the family Juncaceae and the nutrient source and then cultured at a growth temperature for the seed culture of mycelia for a predetermined time period.
