How to log SQL calls with NHibernate to the console of Visual Studio?

I have the following configuration file for NHibernate:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">
  <session-factory>
    <property name="connection.connection_string">Server=.\SQLEXPRESS;Database=mydb;Integrated Security=True;</property>
    <property name="dialect">NHibernate.Dialect.MsSql2005Dialect</property>
    <property name="connection.provider">NHibernate.Connection.DriverConnectionProvider</property>
    <property name="connection.driver_class">NHibernate.Driver.SqlClientDriver</property>
    <property name="connection.release_mode">auto</property>
    <property name="adonet.batch_size">500</property>

    <property name="show_sql">true</property>

  </session-factory>
</hibernate-configuration>

But the SQL doesn't show in the output window of Visual Studio. Is it mandatory to install log4net? Or should show_sql work alone?