A photodetector comprising: a stem having electrical conductivity; a pyroelectric element arranged on front surface side of the stem and including a first light receiving portion and a second light receiving portion that are connected in series and that generate voltages of opposite polarities upon receiving light; a projecting portion (511, 512) that has a shape projecting from a front surface of the stem and that has thermal conductivity; and a circuit board that is in the form of a flat plate and that includes a circuit electrically connected to the pyroelectric element, wherein the circuit board includes: a first conductor pattern electrically connected to the first light receiving portion of the pyroelectric element; and a second conductor pattern electrically connected to the second light receiving portion of the pyroelectric element, the first conductor pattern and the second conductor pattern having zones that are arranged parallel and apart from each other when viewing the circuit board in plan; wherein the pyroelectric element comprises a first connection member and a second connection member, wherein the first light receiving portion of the pyroelectric element is electrically connected to the first conductor pattern through the first connection member, the second light receiving portion of the pyroelectric element is electrically connected to the second conductor pattern through the second connection member, and when viewing the circuit board in plan, the first conductor pattern and the second conductor pattern have zones in which the first conductor pattern and the second conductor pattern are arranged parallel to be kept apart from each other at a middle point between the first connection member and the second connection member. a stem having electrical conductivity; a pyroelectric element arranged on front surface side of the stem and including a first light receiving portion and a second light receiving portion that are connected in series and that generate voltages of opposite polarities upon receiving light; a projecting portion (511, 512) that has a shape projecting from a front surface of the stem and that has thermal conductivity; and a circuit board that is in the form of a flat plate and that includes a circuit electrically connected to the pyroelectric element, wherein the circuit board includes: a first conductor pattern electrically connected to the first light receiving portion of the pyroelectric element; and a second conductor pattern electrically connected to the second light receiving portion of the pyroelectric element, the first conductor pattern and the second conductor pattern having zones that are arranged parallel and apart from each other when viewing the circuit board in plan; wherein the pyroelectric element comprises a first connection member and a second connection member, wherein the first light receiving portion of the pyroelectric element is electrically connected to the first conductor pattern through the first connection member, the second light receiving portion of the pyroelectric element is electrically connected to the second conductor pattern through the second connection member, and when viewing the circuit board in plan, the first conductor pattern and the second conductor pattern have zones in which the first conductor pattern and the second conductor pattern are arranged parallel to be kept apart from each other at a middle point between the first connection member and the second connection member. a first conductor pattern electrically connected to the first light receiving portion of the pyroelectric element; and a second conductor pattern electrically connected to the second light receiving portion of the pyroelectric element, the first conductor pattern and the second conductor pattern having zones that are arranged parallel and apart from each other when viewing the circuit board in plan; wherein the pyroelectric element comprises a first connection member and a second connection member, wherein the first light receiving portion of the pyroelectric element is electrically connected to the first conductor pattern through the first connection member, the second light receiving portion of the pyroelectric element is electrically connected to the second conductor pattern through the second connection member, and when viewing the circuit board in plan, the first conductor pattern and the second conductor pattern have zones in which the first conductor pattern and the second conductor pattern are arranged parallel to be kept apart from each other at a middle point between the first connection member and the second connection member.