A medium passage detection device for optically detecting passage of a medium under transport in a transport path, the medium passage detection device comprising: a light-emitting unit configured to emit a light beam including an S-wave component and a P-wave component to the transport path; an S-wave-extracting unit configured to separate the S-wave component included in the light beam emitted from the light-emitting unit; and a light-receiving unit configured to receive the S-wave component that is passed through the medium under transport in the transport path and separated by the S-wave-extracting unit,and the light beam emitted from the light-emitting unit including an optical axis and, in the transport path, an angle being formed between the optical axis and a direction perpendicular to the transport path, the angle being greater than or equal to 60° and less than 90°, characterised by a detecting unit configured to detect the presence or absence of the medium based on an attenuated amount or attenuation percentage of an output signal from the light-receiving unit, wherein the detecting unit judges that the medium is present - either when an output signal from the light-receiving unit is lower than a percentage of an initial value, - or when an attenuated amount or attenuation percentage of the output signal from the light-receiving unit with respect to the initial value, is not lower than a threshold stored in a memory, where said initial value is an output value from the light-receiving unit at a stage before a medium is under transport in the transport path. a light-emitting unit configured to emit a light beam including an S-wave component and a P-wave component to the transport path; an S-wave-extracting unit configured to separate the S-wave component included in the light beam emitted from the light-emitting unit; and a light-receiving unit configured to receive the S-wave component that is passed through the medium under transport in the transport path and separated by the S-wave-extracting unit,and the light beam emitted from the light-emitting unit including an optical axis and, in the transport path, an angle being formed between the optical axis and a direction perpendicular to the transport path, the angle being greater than or equal to 60° and less than 90°, characterised by a detecting unit configured to detect the presence or absence of the medium based on an attenuated amount or attenuation percentage of an output signal from the light-receiving unit, wherein the detecting unit judges that the medium is present - either when an output signal from the light-receiving unit is lower than a percentage of an initial value, - or when an attenuated amount or attenuation percentage of the output signal from the light-receiving unit with respect to the initial value, is not lower than a threshold stored in a memory, - either when an output signal from the light-receiving unit is lower than a percentage of an initial value, - or when an attenuated amount or attenuation percentage of the output signal from the light-receiving unit with respect to the initial value, is not lower than a threshold stored in a memory, where said initial value is an output value from the light-receiving unit at a stage before a medium is under transport in the transport path.