A recording medium loading mechanism for use in a signal processing apparatus has a motor which operates uni-directionally, and a power transmission mechanism operative to convert the rotation of the motor output shaft into reciprocating movements of a recording medium carrier between a loading position and a recording-reproducing position. The power transmission mechanism includes a crank pin (18) connected to the motor output shaft, a lost-motion connection for converting the rotational motion of the motor output shaft into reciprocal movements to move a reciprocating member (19), and cams cooperative with cam followers to transmit the reciprocating movements to the recording medium carrier.