periods of the supply reel and the tape up reel are t s and t t respectively the transported amount l 2 and the residual amount l 1 of the tape can be computed as follows equ1 where constants k 1 k 1 39 k 2 and k 2 39 are defined by the hub diameter of the tape reel the depth of the tape and the tape speed accordingly if the rotational periods of the reels are detected the transported or residual amount of the tape can be readily computed from the above equations if reproduction in the normal direction is performed with a constant tape speed as denoted in fig1 by a solid line arrow a the rotational periods t 1 and t 2 will respectively correspond to t s in equation 1 and t t in equation 2 for computing the residual amount of tape at the same time in order to compute the transported amount of tape the rotational periods t 1 and t 2 must correspond to t s in equation 3 and t t in equation 4 therefore the transfer switches 8 and 9 are controlled so as to respectively select t 1 and t 2 as t s and t t by an instruction signal of the signal generator 11 for instructing the direction of reproduction or the display of the transported residual amount of tape and then t 1 and t 2 are inputted to the operation circuit.