In this paper, the action of pseudogroup of all point transformations 
on the bundle of equations
$$
  y''=u^0(x,y)+u^1(x,y)y'+u^2(x,y)(y')^2 +u^3(x,y)(y')^3
$$
is investigated.
The 1-st nontrivial differential invariant of this action is 
calculated. This invariant is a horizontal differential 2-form with 
values in some algebra. It is defined on the bundle of 2--jets of 
sections of the considered bundle. It is proved that this form is a 
unique obstruction to linearizability of these equations by point
transformations.