function CheckForm(Join){ 
   var chk1=document.Join.U_checkAgreement1.checked;
   var chk2=document.Join.U_checkAgreement2.checked;
   if(chk1==""){
    alert('약관에동의해 주세요');
    return false;
   }
   if(chk2==""){
    alert('약관에동의해 주세요');
    return false;
   }
}

<form name=Join action=""  onSubmit="return CheckForm(this)">
<input name="U_checkAgreement1" type="checkbox" value="" id="U_checkAgreement1" class="inputCheck" /> 약관동의
<input name="U_checkAgreement2" type="checkbox" value="" id="U_checkAgreement1" class="inputCheck" /> 약관동의
 <input type="image" src="./images/btn_member_join3.gif" border="0" alt="회원가입">
</form>