  Solar inter-network magnetic field is the weakest component of solar
magnetism, but contributes most of the solar surface magnetic flux. The study
on its origin has been constrained by the inadequate tempo-spatial resolution
and sensitivity of polarization observations. With dramatic advances in spatial
resolution and detective sensitivity, solar spectro-polarimetry provided by the
Solar Optical Telescope aboard Hinode in an interval from solar minimum to
maximum of cycle 24 opens an unprecedented opportunity to study the cyclic
behavior of solar inter-network magnetic field. More than 1000 Hinode
magnetograms observed from 2007 January to 2014 August are selected in the
study. It has been found that there is a very slight correlation between
sunspot number and magnetic field at the inter-network flux spectrum. From
solar minimum to maximum of cycle 24, the flux density of solar inter-network
field is invariant, which is 10$\pm1$ G. The observations suggest that the
inter-network magnetic field does not arise from the flux diffusion or flux
recycling of solar active regions, thereby indicating the existence of a
locally small-scale dynamo. Combining the full-disk magnetograms observed by
SOHO/MDI and SDO/HMI in the same period, we find that the area ratio of the
inter-network region to the full-disk of the Sun apparently decreases from
solar minimum to maximum but always exceeds 60\% even though in the phase of
solar maximum.
