To provide a twist frequency determining method of a core capable of suppressing contraction of a timing belt caused by heating, and capable of maintaining performance of the timing belt.

The timing belts of preferred embodiments 1 and 2 and comparative examples 1-3 having difference only in an upper twist frequency of a strand of the core are wrapped around between pulleys in an environment of 100°C, and the constant tension is applied. The timing belt is allowed to stand still in this condition until a prescribed reference time, and the whole length of each of the timing belts is measured during and after the test, and the contraction amount is calculated. When taking the core of the comparative examples 1 and 2 having the upper twist frequency of the strand of 2.0 times/inch as a reference, the contraction amount of the heated timing belt can be suppressed in the preferred embodiments 1 and 2 having the upper twist frequency reduced to 1.5 times/inch, and sufficient bending fatigue resistance can be maintained.