Usando float:left al secondo div che contiene il testo prova,lo puoi allineare orizzontalmente:
codice:
 <style>

   .Div_box {width:800px;heigh:600px;background-color:red }
.image_space {width:500;heigh:600px;float:left; }
.Div_Central_Text  {
float:left;
width:300px;
height : 600px;
background-color:#FFFFFF;
  
text-align : left;
font-size : 12px;
line-height : 50px;

}
  </style>
<body>
<div class="Div_box">
<div class="image_space">[img]images/1.jpg[/img]</div>
<div class="Div_Central_Text">prova</div>


</div>
</body>