A masterbatch pellet is provided for obtaining a polyamide resin composition excellent in mechanical strength and heat aging resistance. The masterbatch pellet is obtained by melt-kneading a raw material component containing a thermoplastic resin (A) and a metal oxide (B), wherein a content of the metal oxide (B) is 0.5% by mass or more and a fraction of the metal oxide (B) after the melt kneading is present as an aggregated particle of 5 μm or more in a major axis length. A proportion of the aggregated particle of 5 μm or more in a major axis length in a whole metal oxide after the melt kneading is preferably 30% by mass or less.