Using the drag formula
Drag = Coefficient of drag*(1/2air density*V squared)*surface area.
The part of the formula I have put in brackets is IAS.
As altitude increases air density decreases.
Therefore assuming Cd and surface area remain the same, for the same amount of drag, as air density decreases the V squared must increase to keep the formula balanced. V in this case is TAS.
So for an increase in altitude at a constant IAS and the same drag, the TAS increases.