The maximum price imposed by the government is at 1.424 euros for a litre of diesel compared to 1.422 euros for a litre of petrol (the most common type, 95 E10). This price may be different at your local filling station, which is not obliged to apply the maximum price, but a turning point has been reached anyway, but the gap is getting smaller anyway.
It's the Belgian government's policy to make diesel more expensive than petrol to discourage people from buying polluting diesel cars. The government has been narrowing the gap slowly through changing excise duties on fuels.
The policy pays off, because fewer and fewer diesel cars are being sold, with the gains going to petrol cars and, to a lesser extent, electric vehicles. Belgium was traditionally a country where many diesel cars were being sold, but now increasing attention is being paid to air pollution and the impact on our health. Cars running on petrol are also polluting our air; environmentalists are pressing for extra measures to give electric cars a boost (running on sustainable energy).