We consider the ordinary differential equation of the second order ẍ+ψ(εt) sin(x-φ(εt))=0 with the coefficients ψ and φ depending slowly on time. By using a Wentzel-Kramers-Brillouin (WKB)-like method we construct two asymptotic series for a general solution of the equation in the limit ε→0 (adiabatic limit). One of them is true when the variable t is far from the zeroes of the coefficient ψ and the other one is valid in the neighborhoods of these these zeroes.