A differential pressure type flowmeter comprising: a main flow channel configured to allow fluid to flow; an orifice provided in the main flow channel ; an upstream pressure sensor configured to detect pressure of fluid on an upstream side of the orifice ; a downstream pressure sensor configured to detect pressure of fluid on a downstream side of the orifice ; a first flow channel communicating with the main flow channel , and configured to guide the fluid on the upstream side of the orifice to the upstream pressure sensor ; and a second flow channel communicating with the main flow channel , and configured to guide the fluid on the downstream side of the orifice to the downstream pressure sensor , wherein the upstream pressure sensor includes a first sensor body that detects pressure of fluid which flows from the main flow channel into a first fluid chamber through the first flow channel , the downstream pressure sensor includes a second sensor body that detects pressure of fluid which flows from the main flow channel into a second fluid chamber through the second flow channel , a first flow channel length of the first flow channel coincide with a second flow channel length of the second flow channel , the first flow channel length being a length from the main flow channel to an inlet of the first fluid chamber , the second flow channel length being a length from the main flow channel to an inlet of the second fluid chamber , a first flow channel diameter of the first flow channel coincide with a second flow channel diameter of the second flow channel , characterised in that : a ratio of the first flow channel length to the first flow channel diameter and a ratio of the second flow channel length to the second flow channel diameter are not less than 20 times and not more than 30 times, respectively. a main flow channel configured to allow fluid to flow; an orifice provided in the main flow channel ; an upstream pressure sensor configured to detect pressure of fluid on an upstream side of the orifice ; a downstream pressure sensor configured to detect pressure of fluid on a downstream side of the orifice ; a first flow channel communicating with the main flow channel , and configured to guide the fluid on the upstream side of the orifice to the upstream pressure sensor ; and a second flow channel communicating with the main flow channel , and configured to guide the fluid on the downstream side of the orifice to the downstream pressure sensor , wherein the upstream pressure sensor includes a first sensor body that detects pressure of fluid which flows from the main flow channel into a first fluid chamber through the first flow channel , the downstream pressure sensor includes a second sensor body that detects pressure of fluid which flows from the main flow channel into a second fluid chamber through the second flow channel , a first flow channel length of the first flow channel coincide with a second flow channel length of the second flow channel , the first flow channel length being a length from the main flow channel to an inlet of the first fluid chamber , the second flow channel length being a length from the main flow channel to an inlet of the second fluid chamber , a first flow channel diameter of the first flow channel coincide with a second flow channel diameter of the second flow channel , characterised in that : a ratio of the first flow channel length to the first flow channel diameter and a ratio of the second flow channel length to the second flow channel diameter are not less than 20 times and not more than 30 times, respectively. <EOS>