An earthquake may be regarded as the abrupt slip of a fault and therefore it is important for seismologists to understand the friction law that governs the fault motion. However, we cannot do experiments using faults, which are generally larger than kilometer scale. Therefore, we have to somehow extrapolate the results of laboratory experiments to the fault scale. To this end, a theoretical consideration is vital because the extrapolation cannot be verified without considering the scale dependence of underlying physical processes. Here we review a recent understanding of such mechanisms behind some standard friction laws known in the field of earthquake physics and discuss its scale dependence.