rotatetomoving.cs

   Vector3 latestPos;

   public void FixedUpdate()
    {   
        //前フレームとの位置の差から進行方向を割り出してその方向に回転します。
        Vector3 differenceDis = new Vector3(transform.position.x, 0, transform.position.z) - new Vector3(latestPos.x, 0, latestPos.z);
        latestPos = transform.position;
        if (Mathf.Abs(differenceDis.x) > 0.001f || Mathf.Abs(differenceDis.z) > 0.001f)
        {
            if (movingDirecion == new Vector3(0, 0, 0)) return;
            Quaternion rot = Quaternion.LookRotation(differenceDis);
            rot = Quaternion.Slerp(rb.transform.rotation, rot, 0.2f);
            this.transform.rotation = rot;
        }
    }