A processing apparatus includes a photosensor, a processing unit and a control unit. The control unit detects a light receiving signal output from the photosensor during a first light emission period and obtains a first detection result, and makes first determination based on the first detection result by determining whether the processing unit can execute a function. The control unit detects the light receiving signal after a waiting period and obtains a second detection result. The control unit makes second determination based on the second detection result by determining whether the processing unit can execute the function. The control unit makes the second determination in a second mode that requires small consumption power, and according to determination in the second determination in the second mode that the processing unit can execute the function, the control unit keeps the second mode and makes the first determination.