I keep my olive during winter inside and temperature is around 13-20 °C and
stands next to window...so has enough light but its leaves keep falling off so
how to stop it?

You are keeping it too warm. Olives need a cool rest in the winter.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)

A three month rest seems to be enough for Olea europaea "little Ollie"
(Montra). Olives need _lots_ of light. Here approx.: May-Nov. outside,
Dec.-April inside under lights. If you can provide cooler night temps.
inside so much the better. Well drained soil.
Asuming nothing else is going on...loss of leaves(that shoudn't drop) is
just a result of low light levels. While evergreen, leaves don't last
forever.;-).

Alan

Blue Mountain Bonsai
Ontario, Canada
Zone 6