A flow velocity determining apparatus comprising: an irradiating unit that is configured to emit irradiation light toward a liquid mixture which contains a non-spherical solid and a liquid and flows through a flow path; a detecting unit that is configured to detect reflected light emitted by the irradiating unit and reflected by the liquid mixture flowing through the flow path; and a determining unit that is configured to determine a flow velocity of the liquid mixture by using a light amount of the reflected light detected by the detecting unit, wherein the detecting unit includes a first detecting unit disposed on one side of an optical axis (L2A) of the reflected light and a second detecting unit disposed on the other side when seen in the plan view of an imaginary plane including an optical axis (L1A) of the irradiation light and the optical axis (L2A) of the reflected light; the determining unit compares a first light amount of the reflected light detected by the first detecting unit with a second light amount of the reflected light detected by the second detecting unit to determine the flow velocity of the liquid mixture; characterized in that the determining unit is configured to determine a flow direction of the liquid mixture as a first direction from one side of the flow path toward the other side of the flow path in a case in which the first light amount is larger than the second light amount and a difference between the first light amount and the second light amount is larger than a reference difference, and is configured to determine the flow direction of the liquid mixture as a second direction opposite to the first direction in a case in which the first light amount is smaller than the second light amount and the difference between the first light amount and the second light amount is larger than the reference difference. an irradiating unit that is configured to emit irradiation light toward a liquid mixture which contains a non-spherical solid and a liquid and flows through a flow path; a detecting unit that is configured to detect reflected light emitted by the irradiating unit and reflected by the liquid mixture flowing through the flow path; and a determining unit that is configured to determine a flow velocity of the liquid mixture by using a light amount of the reflected light detected by the detecting unit, wherein the detecting unit includes a first detecting unit disposed on one side of an optical axis (L2A) of the reflected light and a second detecting unit disposed on the other side when seen in the plan view of an imaginary plane including an optical axis (L1A) of the irradiation light and the optical axis (L2A) of the reflected light; the determining unit compares a first light amount of the reflected light detected by the first detecting unit with a second light amount of the reflected light detected by the second detecting unit to determine the flow velocity of the liquid mixture; characterized in that the determining unit is configured to determine a flow direction of the liquid mixture as a first direction from one side of the flow path toward the other side of the flow path in a case in which the first light amount is larger than the second light amount and a difference between the first light amount and the second light amount is larger than a reference difference, and is configured to determine the flow direction of the liquid mixture as a second direction opposite to the first direction in a case in which the first light amount is smaller than the second light amount and the difference between the first light amount and the second light amount is larger than the reference difference.