  A theory of the mean tilt of sunspot bipolar regions (the angle between a
line connecting the leading and following sunspots and the solar equator) is
developed. A mechanism of formation of the mean tilt is related to the effect
of Coriolis force on meso-scale motions of super-granular convection and
large-scale meridional circulation. The balance between the Coriolis force and
the Lorentz force (the magnetic tension) determines an additional contribution
caused by the large-scale magnetic field to the mean tilt of the sunspot
bipolar regions at low latitudes. The latitudinal dependence of the solar
differential rotation affects the mean tilt which can explain deviations from
the Joy's law for the sunspot bipolar regions at high latitudes. The obtained
theoretical results and performed numerical simulations based on the nonlinear
mean-field dynamo theory which takes into account conservation of the total
magnetic helicity and the budget equation for the evolution of the Wolf number
density, are in agreement with observational data of the mean tilt of sunspot
bipolar regions over individual solar cycles 15 - 24.
