at each of incident positions and each of angles based on outputs of the gamma ray detector 19 the apparatus can be selectively operated in first and second operation modes by the control of the system controller 11 in the first operation mode the gamma rays which are emitted from the plane like source 21 and transmitted through the subject are detected by the gamma ray detector 19 then an absorption coefficient map a c map which is necessary for correcting absorption is generated based on the number of counts the absorption coefficient map is a two dimensional distribution of the section of interest in the second operation mode the gamma rays which are emitted from ri and prescribed to the subject are detected by the gamma ray detector 19 thereby reconstructing the two dimensional distribution showing the number of ris of the section of interest that is emission computed tomogram in the first operation mode transmission data is output from the counting processor 31 in the second operation mode emission data is output from the counting processor 31 transmission data and emission data serving as data showing a count number of of the incident gamma rays of each of the incident positions and that of each of angles can be obtained by the same counting process transmission data has substantially the same meaning as projection data of the x ray ct transmission data corresponds to a set of the number of counts which is counted when the gamma ray detector 19 and the plane like source 21 rotate around the subject the number of counts corresponds to an integral of the absorption coefficient of a tissue of the subject on each of the paths of the gamma rays for example a part of 100 gamma.