A power conversion device comprising: a casing including a housing portion, the housing portion being separated into a first housing chamber and a second housing chamber by a partition wall extending in a vertical direction, one surface of the partition wall being exposed to the first housing chamber side, another surface of the partition wall being exposed to the second housing chamber; a circuit board housed in the first housing chamber, the circuit board including an inverter circuit or an inverter control circuit configured to control the inverter circuit, the circuit board being attached to the one surface of the partition wall; a water-cooling heat dissipation member housed in the second housing chamber and attached to a site in the other surface of the partition wall, the site being on an opposite side of the partition wall from the circuit board, the water-cooling heat dissipation member allowing a coolant to flow inside; a cooling fan configured to generate flowing air flowing in the second housing chamber; and a cooling fan control circuit configured to drive the cooling fan, characterized by further comprising: a heat generation component housed in the second housing chamber and attached to the other surface of the partition wall, the heat generation component forming a circuit jointly with the inverter circuit; and a temperature sensor configured to sense a temperature inside the casing or outside the casing, wherein: the cooling fan control circuit is configured to, if the temperature sensed by the temperature sensor is higher than a predetermined temperature set in advance, turn on the cooling fan; if the sensed temperature is equal to or below the predetermined temperature, the cooling fan control circuit controls the cooling fan so as to turn off the cooling fan or make a speed of the flowing air lower than a speed when the cooling fan is in the on state; and the heat generation component is disposed immediately below the water-cooling heat dissipation member in the vertical direction on the other surface of the partition wall. a casing including a housing portion, the housing portion being separated into a first housing chamber and a second housing chamber by a partition wall extending in a vertical direction, one surface of the partition wall being exposed to the first housing chamber side, another surface of the partition wall being exposed to the second housing chamber; a circuit board housed in the first housing chamber, the circuit board including an inverter circuit or an inverter control circuit configured to control the inverter circuit, the circuit board being attached to the one surface of the partition wall; a water-cooling heat dissipation member housed in the second housing chamber and attached to a site in the other surface of the partition wall, the site being on an opposite side of the partition wall from the circuit board, the water-cooling heat dissipation member allowing a coolant to flow inside; a cooling fan configured to generate flowing air flowing in the second housing chamber; and a cooling fan control circuit configured to drive the cooling fan, characterized by further comprising: a heat generation component housed in the second housing chamber and attached to the other surface of the partition wall, the heat generation component forming a circuit jointly with the inverter circuit; and a temperature sensor configured to sense a temperature inside the casing or outside the casing, wherein: the cooling fan control circuit is configured to, if the temperature sensed by the temperature sensor is higher than a predetermined temperature set in advance, turn on the cooling fan; if the sensed temperature is equal to or below the predetermined temperature, the cooling fan control circuit controls the cooling fan so as to turn off the cooling fan or make a speed of the flowing air lower than a speed when the cooling fan is in the on state; and the heat generation component is disposed immediately below the water-cooling heat dissipation member in the vertical direction on the other surface of the partition wall. a heat generation component housed in the second housing chamber and attached to the other surface of the partition wall, the heat generation component forming a circuit jointly with the inverter circuit; and a temperature sensor configured to sense a temperature inside the casing or outside the casing, wherein: the cooling fan control circuit is configured to, if the temperature sensed by the temperature sensor is higher than a predetermined temperature set in advance, turn on the cooling fan; if the sensed temperature is equal to or below the predetermined temperature, the cooling fan control circuit controls the cooling fan so as to turn off the cooling fan or make a speed of the flowing air lower than a speed when the cooling fan is in the on state; and the heat generation component is disposed immediately below the water-cooling heat dissipation member in the vertical direction on the other surface of the partition wall. the cooling fan control circuit is configured to, if the temperature sensed by the temperature sensor is higher than a predetermined temperature set in advance, turn on the cooling fan; if the sensed temperature is equal to or below the predetermined temperature, the cooling fan control circuit controls the cooling fan so as to turn off the cooling fan or make a speed of the flowing air lower than a speed when the cooling fan is in the on state; and the heat generation component is disposed immediately below the water-cooling heat dissipation member in the vertical direction on the other surface of the partition wall.