The correlation between magnetic helicity injection across the photosphere and soft X-ray activity in the solar corona is statistically investigated for seven active regions appearing in the years 1997-2000. The magnetic helicity flux into the solar corona is analyzed by the induction equation method, using magnetograms observed by the Michelson Doppler Imager on SOHO and by the vector magnetograph at the National Astronomical Observatory of Japan. Soft X-ray activity is evaluated from the data observed by the Yohkoh soft X-ray telescope (SXT). Soft X-ray activity of active regions in nonflare phases is found to correlate better with unsigned magnetic helicity flux than with the simple integration of the magnetic helicity flow. In addition, several magnetic variables, e.g., magnetic flux and electric current flux, are investigated, and it is confirmed that any fluxes given by the area integration of magnetic variables are well correlated with soft X-ray activity. However, for the magnetic helicity flow, not only the whole area flux but also the local intensity correlates well with the soft X-ray intensity. The relation between the spatial structure of the magnetic shear and soft X-ray activity is also investigated, and it is revealed that structural complexity in the magnetic shear tends to increase the efficiency of energy liberation in the solar corona. These results indicate that the magnetic helicity injected from the photosphere is relevant to the heating process in the solar corona, although several magnetic variables, not only magnetic helicity, could be related to that.