Flowmeter

A flowmeter configured to measure a flow rate of a target fluid flowing through a pipe, the flowmeter comprising:

• a hollow housing that includes a first side wall and a second side wall facing each other in a direction intersecting a main flow direction of the target fluid, the housing defining an inlet opening that opens toward an upstream side of the pipe in the main flow direction and an outlet opening that is defined in the first side wall, the target fluid flowing into the housing through the inlet opening and out of the housing through the outlet opening;
• a first passage that is defined in the housing and fluidly connects between the inlet opening and the outlet opening, the target fluid flowing through the first passage;
• a second passage that is defined in the housing and branches off from the first passage, a portion of the target fluid in the first passage flowing into the second passage;
• a detector that is disposed in the second passage and configured to detect a flow rate of the target fluid flowing through the second passage;
• and
• a flat surface that is an outer surface of the first side wall and extends between an upstream end of the first side wall in the main flow direction and the outlet opening along the main flow direction.