$Component経由でIDを取得

参照

<script>
// チェック者名制御
    function checkUserNameNull(checked){
    	if(checked == true){
	    	var selector = '{!$Component.form.block.section.checkName.txtcheckName}';
	    	document.getElementById(selector).value = "";
    	}
    }　
    
    // チェック者名Null制御
    function chengeCheckName(checkName){
    	if(checkName != ""){
    		var checkNull = '{!$Component.form.block.section.checkUser.checkNull}';
    		document.getElementById(checkNull).checked = false;
    	}
    }
</script>

<apex:form id="form">
<apex:pageBlock title="{!$Label.LAB319}" mode="view" id = "block">
<apex:pageBlockSectionItem  id="checkName">
            <!-- チェック者名 -->          
            <apex:outputLabel styleClass="col_label" value="{!$Label.LAB140}" />
            <apex:inputText id = "txtcheckName" value="{!sc.checkName}" onchange="chengeCheckName(this.value)" styleClass="input_txt w450 keyword" html-placeholder="完全一致" >
            </apex:inputText>
         </apex:pageBlockSectionItem>
           <apex:pageBlockSectionItem id = "checkUser">
            <!-- チェック者名NULL -->          
            <apex:outputLabel styleClass="col_label" value="{!$Label.LAB372}" />
            <apex:inputCheckbox id="checkNull" value="{!sc.checkNameNullFlg}" onchange="checkUserNameNull(this.checked)"> 
            </apex:inputCheckbox>
         </apex:pageBlockSectionItem>
</apex:pageBlock> 
</apex:form>