A micro flow sensor, comprising: a flow path through which a fluid to be measured flows; a detection unit having a heater configured to heat the fluid to be measured in the flow path and temperature sensors (32 to 35) configured to measure temperature of a fluid to be measured in the flow path; an arithmetic unit configured to measure a flow rate of the fluid to be measured flowing through the flow path based on the temperature of the fluid to be measured, measured by the temperature sensors (32 to 35); a narrowed portion disposed on an upstream side of the detection unit in the flow path for narrowing the flow path; and a first base plate and a second base plate (20, 50) connected to each other, wherein at least one of the base plates has a groove (13, 21) for defining the flow path, the first base plate has two holes (11, 12) in communication with the flow path, the detection unit is disposed outside the second base plate (20, 50) to overlap the flow path, one of the holes is an inlet for introducing a fluid into the flow path, the narrowed portion includes a stepped portion formed in a boundary between the groove (13, 21) and the inlet, and an opening diameter of each of the two holes (11, 12) on a connection surface of the first base plate with the second base plate (20, 50) is formed so as to be smaller than an opening diameter on the other surface, characterized in that the stepped portion is formed in a boundary between the one of the holes formed in the first base plate and the groove or a tapered surface formed in the second base plate, by an adjusted dimension of the groove or tapered surface or the hole. a flow path through which a fluid to be measured flows; a detection unit having a heater configured to heat the fluid to be measured in the flow path and temperature sensors (32 to 35) configured to measure temperature of a fluid to be measured in the flow path; an arithmetic unit configured to measure a flow rate of the fluid to be measured flowing through the flow path based on the temperature of the fluid to be measured, measured by the temperature sensors (32 to 35); a narrowed portion disposed on an upstream side of the detection unit in the flow path for narrowing the flow path; and a first base plate and a second base plate (20, 50) connected to each other, wherein at least one of the base plates has a groove (13, 21) for defining the flow path, the first base plate has two holes (11, 12) in communication with the flow path, the detection unit is disposed outside the second base plate (20, 50) to overlap the flow path, one of the holes is an inlet for introducing a fluid into the flow path, the narrowed portion includes a stepped portion formed in a boundary between the groove (13, 21) and the inlet, and an opening diameter of each of the two holes (11, 12) on a connection surface of the first base plate with the second base plate (20, 50) is formed so as to be smaller than an opening diameter on the other surface, characterized in that the stepped portion is formed in a boundary between the one of the holes formed in the first base plate and the groove or a tapered surface formed in the second base plate, by an adjusted dimension of the groove or tapered surface or the hole.