Fuses blow to protect wiring from melting from passing too much current.
You probably have a hot wire (an energized wire) chafing against the frame somewhere shorting power to ground.
If you keep replacing the fuse you may be incrementally melting the wire.
If you replace the fuse with a bigger fuse you WILL melt the wire (and maybe worse)
Trace the wiring carefully under strong light and look for bubbled insulation or chafed off insulation against the frame.
Current: 1988 BMW R100GS (the 'numberplate' model)
Past: 1987 Yamaha XT350
1983 Honda CB900F
1980 Honda XL185S
1979 Suzuki GS425E