Provided are a photodiode array and its manufacturing method, which maintain the crystalline quality of an absorption layer formed on a group III-V semiconductor substrate to obtain excellent characteristics, and which improve the crystallinity at the surface of a window layer; an epitaxial wafer used for manufacturing the photodiode array; and a method for manufacturing the epitaxial wafer. A method for manufacturing a photodiode array 1 having a plurality of absorption regions 21, includes the steps of: growing an absorption layer 7 on an n-type InP substrate 3; growing an InP window layer on the absorption layer 7; and diffusing a p-type impurity in regions, in the window layer 11, corresponding to the plurality of absorption regions 21. The window layer 11 is grown by MOVPE using only metal-organic sources, at a growth temperature equal to or lower than that of the absorption layer 7.