A physical quantity measuring device comprising: a cylindrical portion defining an inner space; a first case provided to a first end of the cylindrical portion; a sensor unit housed in the first case, the sensor unit being configured to output a signal corresponding to a physical amount of a target fluid; a second case (4, 40) provided to a second end of the cylindrical portion; a circuit portion (61, 610) housed in the second case (4, 40), the circuit portion (61,610) being configured to calculate the physical amount of the target fluid based on the signal received from the sensor unit; characterized by a first partition member comprising a heat insulating portion disposed to face the sensor unit and configured to restrain a heat of the sensor unit from being transmitted to the inner space of the cylindrical portion and a sealing portion configured to provide a hermetic seal between the heat insulating portion and an inner circumferential surface of the cylindrical portion, the first partition member defining a partition between the sensor unit and the inner space of the cylindrical portion at the first end of the cylindrical portion; and a heat-insulative second partition member (9, 90) that is in contact with an opening edge or the inner circumferential surface of the second end of the cylindrical portion, the second partition member (9, 90) defining a partition between the inner space of the cylindrical portion and the circuit portion (61, 610) at the second end of the cylindrical portion and configured to restrain the heat from being transmitted from the inner space of the cylindrical portion to the circuit portion (61, 610). a cylindrical portion defining an inner space; a first case provided to a first end of the cylindrical portion; a sensor unit housed in the first case, the sensor unit being configured to output a signal corresponding to a physical amount of a target fluid; a second case (4, 40) provided to a second end of the cylindrical portion; a circuit portion (61, 610) housed in the second case (4, 40), the circuit portion (61,610) being configured to calculate the physical amount of the target fluid based on the signal received from the sensor unit; characterized by a first partition member comprising a heat insulating portion disposed to face the sensor unit and configured to restrain a heat of the sensor unit from being transmitted to the inner space of the cylindrical portion and a sealing portion configured to provide a hermetic seal between the heat insulating portion and an inner circumferential surface of the cylindrical portion, the first partition member defining a partition between the sensor unit and the inner space of the cylindrical portion at the first end of the cylindrical portion; and a heat-insulative second partition member (9, 90) that is in contact with an opening edge or the inner circumferential surface of the second end of the cylindrical portion, the second partition member (9, 90) defining a partition between the inner space of the cylindrical portion and the circuit portion (61, 610) at the second end of the cylindrical portion and configured to restrain the heat from being transmitted from the inner space of the cylindrical portion to the circuit portion (61, 610).