PURPOSE: To perform the conversion of a transmission speed of a digital data with good accuracy.

CONSTITUTION: A signal 31 to be converted from a oscillator 30 oscillating the signal to be converted such as a digital audio disc or a tape is applied to a latch circuit 32 and a difference detector 34. In the latch circuit 32, a clock pulse 33 in frequency f1 is applied from the transmitter 30 and a delay data 35 shifted by one phase from the signal 31 is applied to the detector 34. A difference output 37 is multiplied with a coefficient 39 sequentially designated from a multiplication coefficient set counter 38 at a multiplier 36. The result of multiplication 34 is latched to a latch circuit 42 to be an interpolation data 47. The data 47 and the delayed data 35 are added at an addition circuit 46 to be a new converted signal 49. The signal 49 is applied to a latch circuit 48. The latch circuit 48 latches the converted signal 49 inputted by taking a clock pulse 52 in frequency f2 supplied from a receiver 50.