We have developed control electronics to operate flux-locked loop (FLL), and analog signal filters to process FLL outputs for 64-channel Double Relaxation Oscillation SQUID (DROS) magnetocardiography (MCG) system. Control electronics consisting of a preamplifier, an integrator, and a feedback, is compact and low-cost due to larger swing voltage and flux-to-voltage transfer coefficients of DROS than those of dc SQUIDs. Analog signal filter (ASF) serially chained with a high-pass filter having a cut-off frequency of 0.1 Hz, an amplifier having a gain of 100, a low-pass filter of 100 Hz, and a notch filter of 60 Hz makes FLL output suitable for MCG. The noise of a preamplifier in FLL control electronics is $7;nV/{surd};Hz$ at 1 Hz, $1.5;nV/{surd};Hz$ at 100 Hz that contributes $6;fT/{surd};Hz$ at 1 Hz, $1.3;fT/{surd};Hz$ at 100 Hz in readout electronics, and the noise of ASF electronics is $150;{mu}V/{surd};Hz$ equivalent to $0.13;fT/{surd};Hz$ within the range of $1{sim}100;Hz$. When DROSs are connected to readout electronics inside a magnetically shielded room, the noise of 64-channel DROS system is $10;fT/{surd};Hz$ at 1 Hz, $5;fT/{surd};Hz$ at 100 Hz on the average, low enough to measure human MCG.