Abstract: By reducing the number of PD arrays, and by simplifying the configuration of an optical power monitor in a WDM system, a miniaturized, cost reduced optical signal monitoring apparatus, optical system or optical signal monitoring method is provided. An optical power monitor 1 has an optical switch 30 having four input ports 31, a DMUX 2 having 48 output ports, and six CSP type PD array modules 50 each including an 8-channel PD array. The output port 32 of the optical switch 30 having four switchable input ports 31 is optically connected to the input port 21 of the AWG 20. The 48 output ports 22 of the AWG 20 are each optically connected to photosensitive surfaces 53 of the individual PDs included in the CSP type PD array modules 50. The CSP type PD array modules 50 are mounted on the end face of the AWG 20.