A Joint Source-Channel Decoding (JSCD) apparatus, method, necessity judging method, and a receiver includes: a source coding rate change judging unit configured to judge whether a source coding rate of the current frame is the same as the previous frame; a source coding rate eligibility judging unit configured to judge whether the source coding rate of the current frame is less than a predetermined source coding rate threshold; a current frame SIR eligibility judging unit configured to judge whether an SIR of the current frame is lower than a predetermined SIR threshold, a necessity result determining unit configured to determine that a JSCD is necessary, when the source coding rate of the current frame is the same as the previous frame, the source coding rate of the current frame is less than the source coding rate threshold, and the SIR of the current frame is lower than the SIR threshold.