In this paper, we consider a nonhomogeneous differential operator equation of first order
u
′
t
+
A
u
t
=
f
t
. The coefficient operator
A
is linear unbounded and self-adjoint in a Hilbert space. We assume that the operator does not have a fixed sign. We associate to this equation the initial or final conditions
u
0
=
Φ
or
u
T
=
Φ
. We note that the Cauchy problem is severely ill-posed in the sense that the solution if it exists does not depend continuously on the given data. Using a quasi-boundary value method, we obtain an approximate nonlocal problem depending on a small parameter. We show that regularized problem is well-posed and has a strongly solution. Finally, some convergence results are provided.