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HTML

              
                <select name="prefecture">
	<option value="都道府県を選択してください…">都道府県を選択してください…</option>
	<option value="東京都">東京都</option>
	<option value="新潟県">新潟県</option>
</select>

<select name="city">
	<option value="市町村を選択してください…">市町村を選択してください…</option>
	<option value="千代田区" data-prefecture="東京都">千代田区</option>
	<option value="中央区" data-prefecture="東京都">中央区</option>
	<option value="港区" data-prefecture="東京都">港区</option>
	<option value="新宿区" data-prefecture="東京都">新宿区</option>
	<option value="新潟市" data-prefecture="新潟県">新潟市</option>
	<option value="長岡市" data-prefecture="新潟県">長岡市</option>
	<option value="上越市" data-prefecture="新潟県">上越市</option>
</select>
              
            
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CSS

              
                
              
            
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JS

              
                jQuery(document).ready(function($){
	var $pref = $('select[name="prefecture"]');
	var $city = $('select[name="city"]');
	var $cityOpts = $city.children('option');
	var cityFirstText = $cityOpts.first().text();

	$city.children('option:not(:first-child)').hide();

	$pref.on('change', function() {
		var prefName = $(this).val();
		var prefIndex = $pref.prop('selectedIndex');

		$cityOpts.show();

		if(prefIndex !== 0) {
			$cityOpts.each(function() {
				var eachPrefName = $(this).attr('data-prefecture');
				var cityName = $(this).val();
				if(eachPrefName !== prefName && cityName !== cityFirstText) {
					$(this).hide();
				}
			});
		} else {
			$cityOpts.hide();
		}
		$city.val(cityFirstText);
	});
});
              
            
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999px

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