1. A maximum likelihood detector (ML detector) configured to execute maximum likelihood detection (ML detection) according to one of a reception signal and a derivative thereof and according to one of a channel estimation signal and a derivative thereof, the ML detector comprising: a search value selecting circuit configured to select a first-layer search value which is not less than a predetermined threshold; and a maximum likelihood detecting circuit (ML detecting circuit) configured to execute at least the following steps: selecting K first-layer candidate signal values according to the first-layer search value, one of the reception signal and the derivative thereof, and one of the channel estimation signal and the derivative thereof, in which the K is an integer greater than one; calculating K second-layer candidate signal values according to the K first-layer candidate signal values; determining whether to add P second-layer supplemental candidate signal value(s) according to the K second-layer candidate signal values and thereby generating a decision result, in which the P is a positive integer; when the decision result is affirmative, adding the P second-layer supplemental candidate signal value(s), selecting P first-layer supplemental candidate signal values according to the P second-layer supplemental candidate signal value(s), and calculating log likelihood ratios (LLRs) according to the K first-layer candidate signal values, the K second-layer candidate signal values, the P first-layer supplemental candidate signal value(s) and the P second-layer supplemental candidate signal value(s); and when the decision result is negative, calculating LLRs according to the K first-layer candidate signal values and the K second-layer candidate signal values.