Provided is a variable valve assembly for internal combustion engines which is capable of accurately determining whether an input rotational component and an output rotational component are fixed to each other. The variable valve assembly includes an intake camshaft for driving an intake valve and a crankshaft for driving the camshaft. The variable valve assembly has a function for changing the relative rotational phase of the intake camshaft with respect to the crankshaft and a function for fixing the intake camshaft to the crankshaft. The variable valve assembly determines whether the intake camshaft is fixed to the crankshaft, on the basis of a total amount of phase variation HCC or the amount of variation in the relative rotational phase of the intake camshaft with respect to the crankshaft.