A water vapor observing apparatus for calculating an amount of water vapor contained within atmospheric air, comprising: an antenna configured to transmit a first radio transmission wave and a second radio transmission wave having different frequencies; a receiving part configured to receive, as reception waves, reflection waves caused by the transmission waves, and an arithmetic processor configured to calculate an amount of the water vapor in a passing area of the transmission waves based on a first reception information generated from a first reception wave obtained from the first transmission wave, and a second reception information generated from a second reception wave obtained from the second transmission wave, wherein the arithmetic processor calculates the water vapor amount in the passing area of the transmission waves based on a level difference between a first reception signal as the first reception information and a second reception signal as the second reception information, wherein the transmission waves are reflected on and returned from one of a ground surface portion and a water surface after passing through the water vapor, and the first radio transmission wave has a frequency in which an attenuation constant of the electromagnetic wave having this frequency, a portion attributable to water vapor is extremely small, so that the first radio transmission wave may not greatly attenuate due to water vapor even by traveling through the water vapor, and the second radio transmission wave has a frequency in which a portion attributable to water vapor is relatively large, so that the second radio transmission wave may greatly attenuate due to water vapor as it travels through the water vapor. an antenna configured to transmit a first radio transmission wave and a second radio transmission wave having different frequencies; a receiving part configured to receive, as reception waves, reflection waves caused by the transmission waves, and an arithmetic processor configured to calculate an amount of the water vapor in a passing area of the transmission waves based on a first reception information generated from a first reception wave obtained from the first transmission wave, and a second reception information generated from a second reception wave obtained from the second transmission wave, wherein the arithmetic processor calculates the water vapor amount in the passing area of the transmission waves based on a level difference between a first reception signal as the first reception information and a second reception signal as the second reception information, wherein the transmission waves are reflected on and returned from one of a ground surface portion and a water surface after passing through the water vapor, and the first radio transmission wave has a frequency in which an attenuation constant of the electromagnetic wave having this frequency, a portion attributable to water vapor is extremely small, so that the first radio transmission wave may not greatly attenuate due to water vapor even by traveling through the water vapor, and the second radio transmission wave has a frequency in which a portion attributable to water vapor is relatively large, so that the second radio transmission wave may greatly attenuate due to water vapor as it travels through the water vapor. <EOS>