An observation device, comprising: a reference signal generator configured to output a reference signal; a local signal generator configured to generate a local signal based on the reference signal; a mixer configured to mix the local signal and a radio frequency, RF, signal of an observation object to down-convert the RF signal to an intermediate frequency, IF, signal; a detector configured to detect the IF signal, and to generate a detection signal; a variable attenuator, connected between the mixer and the detector, configured to attenuate the IF signal; a calibration information setting module configured to set calibration information from a relationship between a change in an attenuation of the variable attenuator and a change in an intensity of the detection signal; and an observation data generator module configured to generate observation data of the RF signal by using the intensity of the detection signal obtained in a state where the value of the variable attenuator is fixed and the calibration information, wherein: the RF signal of the observation object is generated by electromagnetic waves radiated from an environment causing a constant signal intensity and from water vapor, and the observation data is a difference between the intensity of the RF signal due to the electromagnetic wave radiated from the environment causing a constant signal intensity and the intensity of the RF signal due to the electromagnetic wave radiated from the water vapor. a reference signal generator configured to output a reference signal; a local signal generator configured to generate a local signal based on the reference signal; a mixer configured to mix the local signal and a radio frequency, RF, signal of an observation object to down-convert the RF signal to an intermediate frequency, IF, signal; a detector configured to detect the IF signal, and to generate a detection signal; a variable attenuator, connected between the mixer and the detector, configured to attenuate the IF signal; a calibration information setting module configured to set calibration information from a relationship between a change in an attenuation of the variable attenuator and a change in an intensity of the detection signal; and an observation data generator module configured to generate observation data of the RF signal by using the intensity of the detection signal obtained in a state where the value of the variable attenuator is fixed and the calibration information, wherein: the RF signal of the observation object is generated by electromagnetic waves radiated from an environment causing a constant signal intensity and from water vapor, and the observation data is a difference between the intensity of the RF signal due to the electromagnetic wave radiated from the environment causing a constant signal intensity and the intensity of the RF signal due to the electromagnetic wave radiated from the water vapor. the RF signal of the observation object is generated by electromagnetic waves radiated from an environment causing a constant signal intensity and from water vapor, and the observation data is a difference between the intensity of the RF signal due to the electromagnetic wave radiated from the environment causing a constant signal intensity and the intensity of the RF signal due to the electromagnetic wave radiated from the water vapor.