var head = document.getElementById('char_furuta_head');

function anime() {

var direction = 1; /* 1 = down, -1 = up */

setInterval(frame, 1000);

function frame() {

    if (direction == 1) {

        var lel = head.style.top;

        lel += 2;

        console.log(lel);

        head.style.top = lel + '%';

        if(lel == 20) { direction = -1; };

    } else {

        var lel = head.style.top;

        lel -= 2;

        head.style.top = lel + '%';

        if(lel == 0) { direction = 1; }; 

    }

}

}

​

anime();