In a scrambled signal receiving apparatus connectable to a television receiver equipped with a BS (broadcasting satellite) tuner, the scrambled signal received by a BS antenna and converter 1 is supplied to a scrambled signal decoder 16 and a BPF 4. In the scrambled signal decoder 16, a picture signal is derived and then detected by a decoder output detecting circuit 17. The detection result is supplied to a control microcomputer 15, so that a judgement is made as to whether or not the scrambled signal decoder 16 is in operation. If the scrambled signal decoder 16 is in operation, the changing switches 5 and 12 are set to the side of the scrambled signal decoder 16. If the scrambled signal decoder 16 is not operated, the power supply control signal is sent from the control microcomputer 15, whereby when the power supply of the scrambled signal decoder 16 is not turned ON, the switches 5 and 12 are set to the side of the normal broadcasting system. Accordingly, it is possible to prevent that the scrambled signal is received while the power supply of the scrambled signal decoder is turned OFF.