あ、初期化ミス…↓こうでしたorz

//初期化
static int frame_time = 0
static inttime_know = GetNowCount() - 1;
static inttime_back = GetNowCount() - 1;

//1フレームの時間を求める処理
if(time_back != know_time;)time_back = time_know;
time_know = GetNowCount();
frame_time = time_back - time_know;