A transmission device of the present invention comprises: an orthogonal modulator performing orthogonal modulation on a carrier wave using an input signal comprising an in-phase signal component and an orthogonal signal component, and outputs a modulation signal to be transmitted; a detection unit detecting a carrier leaking component included in the modulation signal; a DC offset value computing unit computing a DC offset value to be added to the input signal for compensating the carrier leaking component, based on the carrier leaking component; and an abnormality judgment unit judging abnormality of the orthogonal modulator based on the DC offset value. By monitoring the DC offset value, a failure of the orthogonal modulator or an indication thereof is detected, and transmission of an abnormal modulation signal including a distortion component is prevented. An increase of the leaking power to adjacent channels, which causes the quality deterioration in adjacent radio communication lines, can be prevented.