A very sensitive thin film sensor was developed using a transmission line. The sensor element consists of a coplanar line fabricated from a ceramic substrate and amorphous-CoNbZr film (4 μm in thickness). The ceramic substrate with high permittivity enhanced the phase change in the carrier, and a phase change around 300 degrees/Oe was achieved. A small phase change was detected using a Dual Mixer Time Difference (DMTD) method when a weak magnetic field was applied. The sensor was applied to magnetocardiogram (MCG) measurement without magnetic shielding. Measurement of the MCG signals at 16 points without magnetic shielding were successful, and these signals were found to roughly agree with MCG signals obtained with a SQUID (Superconducting Quantum Interference Device).