The present invention provides a humidifier capable of preventing water generated by dew condensation of vapor from being combined with air, so that an air humidity can be stably controlled. A humidifier according to the present invention includes a storage tank that stores water, the storage tank being opened upward, a heater that heats the water in the storage tank, and a guide plate that is disposed above the storage tank, on at least one of one side and the other side in a first direction extending along a horizontal direction with respect to a center of the storage tank. The guide plate extends in an inclined manner such that an upper part thereof is positioned closer to the center of the storage tank in the first direction than a lower part thereof.