<apex:page controller="FlowpageController" sidebar="false">
<div style="float: left; width: 400px; padding: 10px;">
  <flow:interview name="CaseFlow" interview="{!caseFlow}" reRender="myPanel" />
</div>
<apex:outputPanel id="myPanel" style="float: left; min-width:400px; padding: 10px;">
  <apex:pageBlock title="ケース内容">
    <apex:pageBlockSection columns="1">
      <apex:outputField value="{!sourceCase.ID}"/>
      <apex:outputField value="{!sourceCase.CaseNumber}"/>
      <apex:outputField value="{!sourceCase.Subject}"/> 
    </apex:pageBlockSection> 
  </apex:pageblock>
</apex:outputPanel>
</apex:page>

public with sharing class FlowpageController {

    public Flow.Interview.CaseFlow caseFlow {get;set;}

    public Case sourceCase{
        get{
            try{
                List<Case> results = [SELECT Id,CaseNumber,Subject FROM Case Where CaseNumber = :caseFlow.vaCaseNo];
                if(results.size() !=0){
                    return results[0];
                }
            }catch(System.QueryException e){
                //just Demo, ignore;
            }
            return new Case();
        }
        set;
    }
    
    public FlowpageController(){
        caseFlow = new Flow.Interview.CaseFlow(new Map<String, Object>());
    }
}