An infrared sensor includes a base substrate, an infrared light receiver, and a beam. The beam includes a separated portion separated from the base substrate to be suspended above the base substrate. The beam is connected at the separated portion to the infrared light receiver. The beam includes a p-type portion containing a p-type semiconductor and an n-type portion containing an n-type semiconductor. The p-type portion has a first three-dimensional structure including first recesses and a first solid portion formed between the first recesses. The first solid portion has, between the first recesses adjacent to each other in plan view, a smallest dimension of less than or equal to 100 nanometers in plan view. The n-type portion has a second three-dimensional structure including second recesses and a second solid portion formed between the second recesses. The second solid portion has, between the second recesses adjacent to each other in plan view, a smallest dimension of less than or equal to 100 nanometers in plan view. The beam satisfies at least one of following conditions (Ia) or (IIa): (Ia) the first solid portion includes a first portion having a Young's modulus of less than or equal to 80% of a Young's modulus of a first reference sample that is made of a material of a type identical to a type of a material constituting the first solid portion and that does not have recesses; and (IIa) the second solid portion includes a second portion having a Young's modulus of less than or equal to 80% of a Young's modulus of a second reference sample that is made of a material of a type identical to a type of a material constituting the second solid portion and that does not have recesses.