Edit in JSFiddle

<table>
  <thead>
    <tr>
      <th></th>
      <th>月</th>
      <th>火</th>
      <th>水</th>
      <th>木</th>
      <th>金</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th>1時限</th>
      <td rowspan="2">国語</td>
      <td>算数</td>
      <td>理科</td>
      <td>社会</td>
      <td>体育</td>
    </tr>
    <tr>
      <th>2時限</th>
      <td>美術</td>
      <td>理科</td>
      <td>社会</td>
      <td>体育</td>
    </tr>
    <tr>
      <th>3時限</th>
      <td>理科</td>
      <td>算数</td>
      <td>理科</td>
      <td>社会</td>
      <td>体育</td>
    </tr>
    <tr>
      <th>4時限</th>
      <td>国語</td>
      <td colspan="2">算数</td>
      <td>社会</td>
      <td>体育</td>
    </tr>
  </tbody>
</table>

table { border-collapse: collapse;}
th,td {
  border: 1px solid #000;
  text-align: center;
}