At a guess it is one of the many varieties of ficus, but that is just a guess.
How long have you had it? When did it start dropping leaves? It looks like it is indoors; is that where it stays all the time? What kind of light does it get.
If it is ficus, they are famous for dropping leaves, when conditions, especially lighting, are not to their liking. The good news of that is since they readily drop leaves, they also readily re-grow them.
Offhand without more information, I think it probably is staying too wet, too much of the time and not getting enough light. It is not in good bonsai soil. Good bonsai soil is very mineral, gritty, free draining, doesn't hold on to water very long. Your soil looks heavy and peat based and wet. How much/ how often do you water?
Indoor trees usually need supplemental lighting, which is a dedicated lamp, just a few inches away, on for 16 hrs a day: