var delta = 500;
        (function () {

            var target = document.querySelector('#slide_wrap_pc');
            var isBottom = function() {
            return window.pageYOffset > window.innerHeight - delta;
            }
            window.addEventListener('scroll', function (e) {
            if (isBottom()) {
                if (!target.classList.contains('slide_b')) {
                target.classList.add('slide_b');
                target.style.display = 'block';
                }
            }
            });
        })();