I’ve found this piece of code and it works:
<?php get_header(); ?>
<div id="job-listings">
<?php
$category = get_the_category();
$theName = $category[0]->name;
$theChild = $category[0]->cat_ID;
$subcats = get_categories('child_of=" . $theChild);
foreach($subcats as $subcat) {
echo "<h3>' . $subcat->cat_name . '</h3>';
echo '<ul>';
$subcat_posts = get_posts('cat=" . $subcat->cat_ID);
foreach($subcat_posts as $subcat_post) {
$postID = $subcat_post->ID;
echo "<li>';
echo '<a href="' . get_permalink($postID) . '">';
echo get_the_title($postID);
echo '</a></li>';
}
echo '</ul>';
} ?>
</div>
<?php get_footer(); ?>