Custom ListView with checkbox changes position while scrolling in android

I am new to android and working on web view demo, I have implemented it. But the problem with me is when I check any check box and scrolling the list view , The checked position changes.

I have tried some links from stack over flow with no luck, SO I request someone can help me to resolve this issue, please My adapter is as below,

adapter

    public class FilterAdapter extends BaseAdapter {

        private static final int TYPE_ITEM = 0;
        private static final int TYPE_SEPARATOR = 1;
        private static final int TYPE_MAX_COUNT = TYPE_SEPARATOR + 1;

        private ArrayList<String> mData = new ArrayList<String>();
        private LayoutInflater mInflater;

        private TreeSet<Integer> mSeparatorsSet = new TreeSet<Integer>();

        public FilterAdapter(Context context) {
            mInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        }

        public void addItem(final String item) {
            mData.add(item);
            notifyDataSetChanged();
        }

        public void addSeparatorItem(final String item) {
            mData.add(item);
            // save separator position
            mSeparatorsSet.add(mData.size() - 1);
            notifyDataSetChanged();
        }

        @Override
        public int getItemViewType(int position) {
            return mSeparatorsSet.contains(position) ? TYPE_SEPARATOR : TYPE_ITEM;
        }

        @Override
        public int getViewTypeCount() {
            return TYPE_MAX_COUNT;
        }

        public int getCount() {
            return mData.size();
        }

        public String getItem(int position) {
            return mData.get(position);
        }

        public long getItemId(int position) {
            return position;
        }

        public View getView(int position, View convertView, ViewGroup parent) {
            ViewHolder holder = null;
            int type = getItemViewType(position);
            System.out.println("getView " + position + " " + convertView + " type = " + type);

            if (convertView == null) {
                holder = new ViewHolder();
                switch (type) {
                    case TYPE_ITEM:
                        convertView = mInflater.inflate(R.layout.raw_filter, null);
                        holder.textView = (TextView) convertView.findViewById(R.id.tv_filter);
                        holder.chk_filter = (CheckBox) convertView.findViewById(R.id.chk_filter);
                        break;
                    case TYPE_SEPARATOR:
                        convertView = mInflater.inflate(R.layout.raw_headr, null);
                        holder.textView = (TextView) convertView.findViewById(R.id.tv_hdr);
                        break;
                }
                convertView.setTag(holder);
            } else {
                holder = (ViewHolder) convertView.getTag();
            }
            holder.textView.setText(mData.get(position));
            return convertView;
        }

}

class ViewHolder {
    public TextView textView;
    public CheckBox chk_filter;
}