an annular clearance 170 between the two housings 160 and 161 when the insulating members 162 and 163 are supported on the respective sides thereof by the first housing 160 and the second housing 161 for connecting the annular clearance 170 with the first recess 156 in the second casing 148 an aperture 171 if formed in the first housing 160 and a lead wire 174 for the diffusion type diaphragm 164 is led to the outside through the aperture 171 the differential pressure transmission block 151 is sandwitched at its supporting diaphragm 169 between the peripheral end of the second casing 148 and the peripheral end of the third casing 149 and disposed within the first recess 156 of the second casing 148 and the second recess 157 of the third casing 159 then sealed fluid such as silicone oil and the like is filled within the first pressure receptive chamber 154 first recess 156 and the first measuring chamber 165 as well as the second pressure receptive chamber 155 second recess 157 and the second measuring chamber 166 by adequate means not shown a channel 172 is formed in the second casing 148 and a lead wire 174 is led to the outside of the apparatus for measuring differential pressure through the channel 172 reference numeral 173 denotes a hermetic seal the operation of the.