var tID = "nav1";
function showmenu(ID){
    if(ID!=tID){
	var IDObj = document.getElementById( ID );
	var tIDObj = document.getElementById( tID );
	var IDObj_content = document.getElementById( ID + "_" );
	var tIDObj_content = document.getElementById( tID + "_" );
    	IDObj.className='nav_link_02';
    	tIDObj.className='nav_link_01';
  	tIDObj_content.style.display='none';
  	IDObj_content.style.display='';
    	tID=ID;
    }
}