We have installed a 61-channel magnetocardiography (MCG) system inside a magnetically shielded room (MSR) with a size of $2.4;m;{ imes}2.4;m;{ imes}2.4;m$. The MCG system consists of 1st-order axial gradiometers containing double relaxation oscillation SQUIDs (DROSs) with pick-up coils of a base line of 70 mm. The MSR holds a shielding factor of 50 at 0.1 Hz and 10000 at 100 Hz, when its door in the middle on a front wall is closed. On opening the MSR door, we have obtained the characteristics of the MCG system with a 2.9 Hz noise generated from an air conditioning unit at 13 m distance off the MSR. In an open-door MSR ($140^{circ}$ opening), a noise at the center channel increases up to $700;fT/Hz^{l/2}$ at 2.9 Hz and $1.7;pT/Hz^{1/2}$ at 60 Hz. MCG signals for a healthy human do not show distortion until the door opens to $45^{circ}$, but show the effect of noise when the door opens further at $90^{circ}$ and $140^{circ}$. With the door opens to $45^{circ}$, MCG measurement can be performed with ease of door operation and without creating claustrophobia for the patient.