An infrared solid state imaging device comprising a substrate, a first PN junction diode and a second PN junction diode connected in series that are provided to be thermally isolated from the substrate, the first PN junction diode and the second PN junction diode converting a temperature change caused by heat converted from emitted infrared rays into an electrical signal, wherein the infrared solid state imaging device comprises: a first region having a first conductivity type; a second region having a second conductivity type which constitutes the first PN junction diode together with the first region, which is in contact with the first region at a first junction surface, and which has a first shortest length (Ln) that is a shortest length from the first junction surface to a second junction surface; a third region having the first conductivity type which is in contact with the second region at the second junction surface, and which has a second shortest length (Lp) that is a shortest length from the second junction surface to a third junction surface, the second shortest length (Lp) being different from the first shortest length (Ln); a fourth region having the second conductivity type which constitutes the second PN junction diode together with the third region, and which is in contact with the third region at the third junction surface; an element isolation region which establishes electrical isolation between the first region and the fourth region, between the first region and the third region, and between the second region and the fourth region; and a metal wire provided on the second region and the third region for electrically connecting the first PN junction diode and the second PN junction diode in series, wherein the shortest length is the length of a shortest path along which holes or electrons can pass through each region when the holes or the electrons injected from a region adjacent on one side pass through each region and reach a region adjacent on the other side, wherein the first conductivity type is P type and the second conductivity type is N type, and the first shortest length (Ln) and the second shortest length (Lp) are set proportional to diffusion lengths of carriers wherein the first region, the second region, the third region, and the fourth region are placed such that the first PN junction diode and the second PN junction diode as a whole have a U shape or L shape in plan view, wherein the first shortest length (Ln) and the second shortest length (Lp) satisfy the following relation: L p L n = μ e τ e μ h τ h where L n is the first shortest length (Ln), L p is the second shortest length (Lp), μ e is mobility of electrons in the third region having the first conductivity type, τ e is a time constant of recombination of the electrons, μ h is mobility of holes in the second region having the second conductivity type, and τ h is a time constant of recombination of the holes. a first region having a first conductivity type; a second region having a second conductivity type which constitutes the first PN junction diode together with the first region, which is in contact with the first region at a first junction surface, and which has a first shortest length (Ln) that is a shortest length from the first junction surface to a second junction surface; a third region having the first conductivity type which is in contact with the second region at the second junction surface, and which has a second shortest length (Lp) that is a shortest length from the second junction surface to a third junction surface, the second shortest length (Lp) being different from the first shortest length (Ln); a fourth region having the second conductivity type which constitutes the second PN junction diode together with the third region, and which is in contact with the third region at the third junction surface; an element isolation region which establishes electrical isolation between the first region and the fourth region, between the first region and the third region, and between the second region and the fourth region; and a metal wire provided on the second region and the third region for electrically connecting the first PN junction diode and the second PN junction diode in series, wherein the shortest length is the length of a shortest path along which holes or electrons can pass through each region when the holes or the electrons injected from a region adjacent on one side pass through each region and reach a region adjacent on the other side, wherein the first conductivity type is P type and the second conductivity type is N type, and the first shortest length (Ln) and the second shortest length (Lp) are set proportional to diffusion lengths of carriers wherein the first region, the second region, the third region, and the fourth region are placed such that the first PN junction diode and the second PN junction diode as a whole have a U shape or L shape in plan view, wherein the first shortest length (Ln) and the second shortest length (Lp) satisfy the following relation: L p L n = μ e τ e μ h τ h where L n is the first shortest length (Ln), L p is the second shortest length (Lp), μ e is mobility of electrons in the third region having the first conductivity type, τ e is a time constant of recombination of the electrons, μ h is mobility of holes in the second region having the second conductivity type, and τ h is a time constant of recombination of the holes.